A technique of short-term tests in vitro by means of the incubation of tumor cell suspensions is utilized as a radioactive-biochemical method for pretherapeutic determination of the resistance in human cancers of the breast. Cell suspensions from primary tumors and metastases reveal individually different responses to cytostatics in vitro. It is possible, therewith, to differentiate two tumor collectives related to in vivo resistant or in vivo sensitive tumors. The responses of the primary lesion and the axillary lymphatic metastasis of the same carcinoma may in single cases also differ in vitro, according to clinical experience with the therapy of breast cancer. A distinct relation can be shown between the histological type of a carcinoma and its in vitro capacity of resistance.