A multidrug-resistant (MDR) cell line isolated from HOB1 lymphoma cells was characterized. The MDR phenotype in this cell line was typified by resistance to vincristine with varying degrees of cross-resistance to Adriamycin, colcemid and actinomycin D. Decreased intracellular [3H]vincristine with concurrent increase in the expression of a 170-kDa membrane glycoprotein (P-glycoprotein) suggested a plausible underlying mechanism for the development of resistance. Amplification of the mdr1gene as well as a homogeneous staining region on the long arm of the 7th chromosome was observed. Moreover, metabolic studies with [14C]glucosamine or [14C]mannose indicated differential expressions of membrane glycoproteins between the drug-sensitive parental and drug-resistant descendant cells. It is concluded that the development of drug resistance in HOB1 lymphoma cells was strongly correlated with the overexpression of P-glycoprotein.