One primary and one serially transplanted methylcholanthrene‐induced mouse sarcoma was studied for the existence of cell clones that differ in their sensitivity to cytostatic drugs. Two transplantation methods were compared. One method was to graft a small sarcoma piece subcutaneously, a second method was to inject a small volume of a cell suspension prepared from the sarcoma. After a growth period, the transplanted tumours were tested for cytostatic sensitivityin vitro.Reduction in H3‐TdR incorporation measured the effect of the cytostatic drug added. The tumours obtained in the recipient mice differed in sensitivityin vitro.Tumours obtained by grafting material from the serially transplanted sarcoma did not differ in sensitivity more than those obtained by injection transplantation. Tumours obtained by grafting pieces of the primary sarcoma showed a significantly more marked degree of diversity than the tumours obtained by injection transplantation of that sarc