Chlorambucil (CLB) uptake by chronic lymphocytic leukemia lymphocytes was studied using a radiometric and a newly developed high-performance liquid chromatography assay. CLB labeled with 14C in either the chloroethyl group or phenyl ring was used with identical results. Drug accumulation by the cells was found to peak at 30 s, was independent of temperature, and was proportional to medium CLB concentration over a wide range. Efflux from cells loaded with CLB and resuspended in drug-free medium was nearly complete at 30 s. The metabolic inhibitors 2-deoxyglucose and NaN3, the nitrogen mustard transport inhibitor hemicholinium-3, and another alkylating agent, melphalan, had no effect on drug uptake. We conclude that CLB enters and exits chronic lymphocytic leukemia lymphocytes by simple diffusion. Cells from 17 patients with all stages of chronic lymphocytic leukemia were studied including three with CLB-resistant disease, and no heterogeneity was found in the peak cell-associated CLB content or in metabolite pattern on high-performance liquid chromatography. These findings make it unlikely that transport or cellular drug metabolism are factors in drug resistance. Drug-DNA binding was found to be temperature-sensitive and increased with time of incubation. Gel filtration of DNA before and after enzymatic digestion indicated the presence of drug-DNA adducts. High-performance liquid chromatography analysis of digested DNA and DNA treated by neutral thermal hydrolysis suggested the presence of multiple adducts. Most of the radioactivity was found as purine adducts. Studies with CLB labeled at two different sites revealed the presence of the phenyl group and ethyl chains in the adducts. A survey of patients showed increased drug-DNA binding in cells from patients with clinical CLB resistance.