Abstract
Cells of the methylene dimethanesulphonate-(MDMS)-resistant Yoshida sarcoma cell line contain a low molecular weight "resistance factor" which is present in the culture medium of these cells and may be utilized by MDMS-sensitive Yoshida sarcoma cells either by co-culturing the two cell lines or by culturing the MDMS-sensitive Yoshida cells in a medium containing 20% used medium of MDMS-resistant Yoshida cells or in the presence of dialysed medium from resistant cells. The "resistance factor" does not inactivate the drug itself or its metabolites, and it has no influence on the sensitivity of the cells if added after MDMS treatment. Twenty-four hours seems to be enough time for the transfer of the resistance factor, but its effect on whole populations decreases within 24 hours of ceasing the supply. The relationship between these findings and the known phenomena of metabolic co-operation are discussed.