The effect of a large number of cytostatic drugs was determined on P-815 mastocytoma. The use of the mastocytoma P-815 in suspension for the evaluation of cytostatic effects offers real technical advantages: the tissue substrate can be cultivated on chemically defined media (with the addition of horse serum), growth-inhibiting effects can be measured by counting cells in the hemocytometer and the cytotoxic concentration can be determined by adding a pH indicator to the medium.