Multidrug resistance (MDR) in cultured human cells is caused by the overexpression of the MDR-1 gene. This gene codes for P-glycoprotein, a proposed ATP-dependent drug efflux pump, which reduces the net intracellular accumulation of a large group of chemotherapeutic agents in resistant cells. We have measured the level of expression of the human MDR-1 gene in a series of patients with chronic lymphocytic leukaemia (CLL). Forty-eight patients included in the study were at different stages of disease and were either untreated or had been treated with alkylating agents or alkylating agents in combination with drugs of the MDR spectrum, and were tested over a period of 3 years. The level of MDR-1 expression was monitored by Northern blotting analysis using a specific cDNA hybridization probe and also after polymerase chain reaction (PCR) amplification of MDR-1 complementary DNA (cDNA). Four of 28 previously untreated patients showed intrinsically high levels of MDR-1 mRNA while 5/19 treated patients had elevated MDR-1 expression. Elevated MDR-1 expression in treated patients was unrelated to the type of chemotherapy and was independent of previous exposure to drugs of the MDR spectrum. Intrinsic MDR-1 gene expression in positive patients did not appear to influence their response to chemotherapy with non-MDR drugs such as alkylating agents.