This report has been prepared as a mini-review of the studies and data, accumulated by a number of investigators since 1978, that are relevant to the 6-day subrenal capsule assay (SRCA) as a predictive in vivo test system. Stressing the need to maintain simplicity and economy, the data reviewed are based on the use of normal, immunocompetent mice and a simple tumour size parameter for evaluating drug activity within a 6-day time frame. Both the laboratory and clinical data that are presented and evaluated support the 6-day SRCA as a predictive test system at the preclinical and clinical levels of drug development and treatment. The question, do host responses create an artifact in the tumour size parameter, is addressed with experimental data illustrating the sensitivity of tumour xenografts, prepared from endocrine target tissues, to respond to biological effects in the 6-day SRCA. SRCA data are also presented suggesting that tumour heterogeneity does not automatically preclude the usefulness of predictive assays based upon a tumour sample.