Cellular resistances to the toxic effects of two environmentally and industrially important platinum (Pt) complexes (K2PtCl6and Pt(SO4)2) were induced separately in Chinese hamster ovary cells (CHO‐S) by continuous exposure to the compounds for 5 and 4 months, respectively. The two resulting cell lines, 220R(resistant to 220 μM K2PtCl6) and 550R(resistant to 550 μM Pt(SO4)2) have resistant phenotypes which are stable for at least 55 population doublings in the absence of a selective agent (i.e., Pt). Each resistance is quite specific with respect to the Pt atom as well as the attached ligands, but both cell lines show some cross resistance to other Pt complexes and to one other Noble metal complex (K3RhCl6). X‐Ray fluorescence determinations of Pt in parental CHO‐S and 220Rcells cultured in the presence and absence of K2PtCl6demonstrated that very little of this compound is taken up by either cell line. However, comparable analyses revealed that the amount of Pt accumulated by 550 cells grown in Pt(SO4)2is 3–4 times the amount taken up by the parental CHO‐S cells. Cell fusions between each resistant cell line and the Pt‐sensitive CHO‐S line yielded hybrids which, in each case, display Pt resistances that are intermediate between those of the resistant and sensitive cells employed in the hybridizations. The resistant phenotypes are therefore codominantly regulated.