Specimens of DMBA‐induced mammary tumour, DMBA‐induced subcutaneous sarcomas, and virus induced sarcomas in the rat were incubated for 24–72 hours as organ cultures. The effect of various concentrations of testosterone, progesterone, and 17‐β‐oestradiol on cell survival and multiplication was measured as changes in incorporation of H3‐TdR into DNA. All three types of tumour reacted to the hormones. Statistical analysis of data demonstrated possible influence of factors other than hormone treatment. The specificity of hormone sensitivity assaysin vitroi