The antiproliferative properties of the osmium(II) complexes cis,fac-[OsIICl2(DMSO)3(L)] and trans,cis,cis-[OsIICl2(DMSO)2(L)2] (L=1H-pyrazole, 1H-imidazole) were studied in three human cancer cell lines, namely 41M (ovary), SK-BR-3 (breast), and SW480 (colon). Their activities were compared with those of osmium(III) and ruthenium(III) NAMI-A type complexes on HT-29 (colon) and SK-BR-3 cancer cell lines. While IC50 values of all the OsII complexes were found to be >1000 μM in all cell lines, Os and Ru-NAMI-A type complexes showed remarkable antiproliferative activity. The marginal in vitro cytotoxicity of the OsII compounds is presumably attributed to their resistance to hydrolysis. However, the Os-NAMI-A complexes, which are also kinetically stable in aqueous solution, showed reasonable antiproliferative activity in vitro when compared with the analogous Ru compounds and with the OsII-DMSO-azole species, indicating that hydrolysis might be not a prerequisite for the antitumor activity of Os-NAMI-A type complexes.