The potential of evaluating the preclinical response of solid tumours was studied in human tumour xenografts in the clonogenic assay. Tumour specimens surgically removed from cancer patients were implanted subcutaneously into thymus-aplastic nude mice. Chemosensitivity of the mouse-grown tumours was tested with a modification of the double-layer soft-agar clonogenic assay. Tumour cells were tested against thirteen established cytostatic drugs at two dosages by continuous exposure. 62 retrospective in vivo/in vitro correlations were done. The clonogenic assay predicted correctly for clinical response in 16/27 (59%) and for resistance in 32/35 (91%). These correlation rates were similar to reported data for fresh solid human tumour specimens. The results support the clinical relevance of the nude mouse/clonogenic assay model.