This study used a co-culture system with Transwell tissue-culture inserts to investigate the role of primary cultures of rat peritoneal mesothelial cells on the proliferation of rat colon-carcinoma cells (CC531 cells). Mesothelial cells significantly inhibited the growth of CC531 cells, while, conversely, CC531 cells stimulated the growth of mesothelial cells. Receptor-binding studies demonstrated the presence of high-affinity IGF-I receptors on the mesothelial and CC531 cells. Both cell types also produced IGF-I, as measured by radioimmunoassay. IGF-I stimulated DNA synthesis in mesothelial cells, but had no effect on the growth of CC531 cells. In co-culture, it was found that IGF-I potentiated the inhibitory effect of mesothelial cells on CC531 cells. The effect of IGF-I on mesothelial-cell proliferation was additive to the stimulatory effect of CC531 cells. TGF-beta had no effect on the growth of the CC531 cells, suggesting that this growth (-inhibitory) factor is not involved in the inhibitory effect of mesothelial cells on CC531 cell growth. The study provides evidence for the existence of a paracrine loop between mesothelial and colon-carcinoma cells, giving more insight into the basic cellular mechanisms that may modulate the growth of intraperitoneal colon carcinoma. Inhibition of CC531-cell proliferation by rat mesothelial cells might explain the earlier finding that tumour cells grow poorly in a surgically uncompromised abdomen.