Previous investigations indicate that some of the metabolites of the hemorheological agent pentoxifylline (PTX), namely 1-(5-hydroxyhexyl)-3,7-dimethylxanthine (M1), 1-(4-carboxybutyl)-3,7-dimethylxanthine (M4) and 1-(3-carboxypropyl)-3,7-dimethylxanthine (M5), concur to some of the biological effects of the drug. However, information on the bioactivity of the major circulating oxidative metabolites of PTX (M4 and M5) is scanty. Here, we compared the effects of M4 and M5 with that of PTX and its major reductive metabolite, M1, on TNF-alpha production and cytotoxicity, endothelial cell proliferation and on the ATPase activity related to some ATP-binding cassette (ABC) transporters. Unlike PTX and M1, M4 and M5 poorly inhibited lipopolysaccaride-stimulated tumor necrosis factor-alpha (TNF-alpha) release by RAW 264.7 murine macrophages, and did not affect at all cell proliferation and upregulation of TNF-alpha-induced vascular cell adhesion molecule-1 (VCAM-1) in H5V endothelioma cells. By contrast, M4 and M5 were more effective than PTX and M1 in protecting WC/1 murine fibrosarcoma cells from TNF-alpha cytotoxicity. Moreover, results from ATP hydrolase assays indicated that neither PTX nor its tested metabolites interacted significantly with the human multidrug resistance transporters p-glycoprotein/multidrug resistance 1 (MDR1), multidrug resistance-related protein 1 (MRP1), and breast cancer resistance protein (BCRP). Based on these results and literature data, M5, retaining some of the PTX effects but lacking in significant inhibition of TNF-alpha production, may be a promising candidate drug for certain pathologic conditions.