In this study, we observed the influence of bovine calf serum (BCS) on the intracellular accumulation of adriamycin (ADR) in Ehrlich ascites tumor cells (wild type EAT cells) and their ADR-resistant strain. The ADR accumulation treated in the medium not containing BCS was S-fold of that containing 10% BCS. ADR incorporated into cells was not effluxed in the absence of the BCS, while it was rapidly effluxed for 30 minutes to about 20% of the amount incorporated into the cells in the presence of 20% BCS in both strains. This suggests that BCS remarkably decreases the ADR accumulation due to the promotion of ADR efflux. Further; the ADR accumulation dea eased with increasing concentrations of BCS in both strains. This indicates that BCS promotes the ADR efflux, dependeing on its concentrations. BCS was heated at 100 degrees C for 7 minutes and its protein contents were degenerated. Irs influence in the ADR accumulation of wild type and ADR-resistant strain was apparent. The ADR accumulation decreased with increasing concentrations of heated BCS in both strains, as well as in the untreated BCS. Thus, the protein content of the BCS did not affect ADR accumulation. Therefore, it is necessary to investigate which components of BCS affect the promotion of ADR efflux.