Overexpression of P-glycoprotein (P-GP) accompanied by multidrug resistance (MDR) to diverse groups of cytostatics was developed by long-term adaptation of mouse leukemic cell line L1210 to vincristine. Two resistant sublines of cells characterized by ID50 values for vincristine 1.05 mg/l (L1210/VCR-1) and 2.3 mg/l (L1210/VCR-2), respectively, were used. The sensitive parental cell line L1210 had the ID50 value for vincristine around 0.01 mg/l. Overexpression of P-GP induced by the adaptation procedure was found to be accompanied by an increase in the mean cell diameter from 10.28 +/- 1.60 microns (mean +/- SD, n = 122) for sensitive L1210 cells to 17.82 +/- 2.59 microns (n = 120) and 37.26 +/- 5.72 microns (n = 121) for L1210/VCR-1 and L1210/VCR-2 resistant cell sublines, respectively. Significant decrease in ability to accumulate [3H]-vincristine from cultivation medium was observed for both resistant cell sublines in comparison to sensitive cells. Accumulation of [3H]-vincristine by sensitive cells is secured only by passive diffusion of the drug across the plasma membrane. Contrary to that, active efflux of drug operating against its diffusion across the plasma membrane should be assumed as a factor influencing the [3H]-vincristine accumulation by resistant cells. Indeed, the time dependence of [3H]-vincristine accumulation by sensitive cells could be fitted using simple monoexponential kinetic dependence in contrast to biexponential kinetic dependences that are necessary for fitting [3H]-vincristine accumulation by both resistant cell sublines. Kinetic analysis of the experimental data indicates that accumulation of [3H]-vincristine by sensitive cells grows to a plateau reflecting probably the equilibrium of drug concentration in the intracellular and extracellular space. On the contrary, accumulation of [3H]-vincristine by both resistant cell sublines was stabilized after an initial growth on a considerably lower level than it was observed for the sensitive cells in the equilibrium.