Recently, it was shown that hepatocyte DNA synthesis after partial hepatectomy (PH) is impaired in interleukin-6-deficient (IL-6(-/-)) mice, which results in significantly delayed, but eventual, recovery of normal liver weight, compared with the IL-6(+/+) controls. Four possible compensatory mechanisms might explain this phenomenon: 1) hepatocyte hypertrophy; 2) activation of the oval cell compartment and subsequent maturation to hepatocytes; 3) non-oval biliary epithelial cell (BEC) proliferation; and/or 4) differential rates of apoptotic cell death in the regenerating liver. These hypotheses were tested by subjecting IL-6(-/-) and IL-6(+/+) mice to PH and determining sequential liver weight, histology, hepatocyte and BEC 5'-bromo-2'-deoxyuridine (BrdU) labeling, liver DNA content, alpha-fetoprotein (AFP) mRNA production, and apoptosis at several time points after PH. Consistent with previous studies, we show that the absence of IL-6 significantly impairs hepatocyte DNA synthesis and delays liver weight recovery after PH, but the defect observed in this study is less severe than that previously reported, and no excess mortality, massive necrosis on histology, nor differences in liver injury test are seen. Interestingly, the IL-6(-/-) mice show more hepatocyte BrdU pulse labeling than the IL-6(+/+) controls at 24 hours, but less at 36, 48, and 60 hours. Continuous BrdU infusion up to 60 hours after PH showed a cumulative hepatocyte labeling index of 79.5% in IL-6(+/+) mice and 70.8% in IL-6(-/-) mice, respectively (P <.03). However, despite a lower labeling index and significantly delayed weight recovery, hepatic mass was equally restored in the two groups by 96 hours. There was no evidence of oval cell proliferation in the IL-6(-/-) mice, as determined by routine histology and AFP mRNA analysis, and non-oval BEC proliferation was also slightly impaired in the IL-6(-/-) mice compared with the IL-6(+/+) mice. In addition, liver DNA content per gram of liver showed an increase compared with normal at 60 hours in both groups, but by 96 hours, there was no difference between the two groups. Thus, neither oval cell nor BEC proliferation, nor hepatocyte hypertrophy, could account for the eventual equivalent weight recovery. There was, however, a difference between the two groups in the rate of apoptosis. In normal livers of both IL-6(-/-) and IL-6(+/+) mice, apoptotic cells were uncommon, and even fewer such cells were detected at 24, 36, and 48 hours after PH. Between 60 and 96 hours after PH, a wave of apoptosis spread through the livers of both groups. The number of apoptotic cells was directly proportional to the magnitude of hepatocyte BrdU labeling and liver DNA content after PH, and the difference between the nadir of apoptosis at 24 hours and the peak at 96 hours was greater for the IL-6(+/+) mice. In addition, a direct comparison between the two groups at 96 hours showed that hepatocyte apoptosis was significantly lower in the IL-6(-/-) versus the IL-6(+/+) mice (P <. 02). Treatment of the IL-6(-/-) mice with rIL-6 completely reversed the hepatocyte proliferation defect and increased the subsequent level of total apoptotic bodies. The fine control of liver weight recovery during regeneration after PH is a complex process that involves both mitosis and apoptosis. IL-6 affects this process by recruiting, and possibly synchronizing, the entry of hepatocytes into cell cycling, which quickly restores liver mass. However, this robust response generates superfluous hepatocytes, which are eliminated via apoptosis, similar to many other processes involving organ growth.