In the subrenal capsule assay, the immuno‐competent mice used have a host immune response to the graft which may introduce a bias in the determination of tumour size. A study was therefore conducted to verify the quality of the correlation that could exist between “macroscopical” and “microscopical” evaluation. A histological study of 12 different tumours treated with cis‐Pt, L‐PAM, DTIC, vindesine, BCNU or TGU demonstrated that a cellular host immune response was observed in 11 out of the 12 control groups and was uneven in the treated groups. The “microscopical” and “macroscopical” evaluations of tumour‐take were 100 and 95% respectively. Although there was a fair correlation between the “microscopic” and “macroscopic” parameters, tumour variations in absence of host reaction ‐ which reflect pure chemosensitivity of the treated tissue ‐ could be measured in 50% of the cases. It was possible to determine abase‐line for rejection of the test results when the control group showed a decrease in mean tumour size exceeding 20% of its initial size on day 0. If the decrease is less than 20%, then the histological evaluation appears to be of considerable additional value.