P-glycoprotein gene (mdrl) amplification and expression were examined in murine leukaemia P388/DX and melanoma B16VDXR cell lines, which exhibit a high level of resistance to a selecting agent, doxorubicin, and express a multidrug-resistant phenotype because they are cross-resistant to multiple cytotoxic drugs. The multidrug-resistant phenotype was obtained in different conditions of selection (in vivo and in vitro for P388/DX and B16VDXR, respectively). In both multidrug-resistant cell lines, an increased expression of P-glycoprotein gene (5 kb transcript detected in Northern blots) was observed and the level of P-glycoprotein mRNA correlated with the degree of resistance. In addition, high molecular weight mRNAs homologous to mdrl gene sequence were consistently detected only in P388/DX cells. Overexpression was associated with a high level of gene amplification only in resistant melanoma cells, whereas it occurred in P388/DX cells with a marginal increase in gene copy number. These results, suggesting that different genetic mechanisms could be responsible for P-glycoprotein overexpression, emphasise the complexity of genetic regulation that may affect tumour cell sensitivity to cytotoxic agents.