The influence of free and N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer-bound adriamycin (ADR) on the induction of multidrug resistance in the A2780 human ovarian carcinoma cell line was studied in vitro. It was found that chronic exposure to free ADR led to an increase in resistance to ADR and Taxol and overexpression of the MDR1 gene. No significant changes in the expression of the MRP gene were found during adaptation to free ADR. In addition to MDR1 gene-encoded multidrug resistance, a significant increase in the resistance against ADR was found before the overexpression of the MDR1 gene was measurable. This non-P-glycoprotein resistance does not appear to be connected with MRP gene-encoded resistance. During adaptation to free ADR, changes in cellular metabolism such as increased rate of glucose uptake, oxidation and glycolysis were detected. Adapted sensitive A2780 cells expressed the MDR1 gene and possessed almost the same decreased sensitivity toward ADR as the ADR-resistant human ovarian carcinoma A2780/AD cells. However, they significantly differed in proliferation rate, cellular metabolism and MRP gene expression. On the contrary, multidrug resistance was not induced after repeated exposure of sensitive A2780 cells to HPMA copolymer-bound adriamycin. The cells did not express the MDR1 gene, the expression of the MRP gene was partially inhibited, and the resistance against Taxol was decreased. Differences were also observed in metabolic changes. In summary, the data indicate that, contrary to free ADR, HPMA copolymer-bound ADR does not induce multidrug resistance in A2780 cell culture after repeated exposure.