This study compared the histological changes induced by human bladder tumor xenografts following their implantation in the subrenal capsular space of immunocompetent mice (CDF1, 6-10 weeks age). Ten human bladder tumors were studied by light microscopic and electron microscopic methods. All materials were obtained from cold-cup biopsy using endoscopic procedure. Single fragments (1 mm X 1 mm X 1 mm) were implanted on Day 0 under the renal capsule of each mouse. Animals were serially sacrificed from day 2 to day 6 and in some cases as late as day 10. All kidneys were sectioned up to 3 levels at the xenograft and stained with appropriate histological reagents. The results of this study were; 1) infiltration of the tumor with mononuclear cells and neutrophils began on day 4, 2) pretreatment with 200 mg/kg cyclophosphamide before implantation reduced cell infiltration into the xenograft, 3) complete rejection of the tumor on day 8, 4) electron microscopic study showed minimum changes of intracellular organs on day 2, 5) but microscopic architecture was preserved in the tumor explants removed on day 4. This morphological study suggested that histological evaluation on day 4 might be a suitable method of subrenal capsule assay using immunocompetent mice. We therefore tested genitourinary carcinomas to determine their sensitivities to commonly use chemotherapeutic agents using histological criteria. Groups of tumor-bearing mice were treated daily for 3 days. Sensitivities for assays were assessed on day 4 by comparison of histological score. Of the 15 cases evaluated, 20% demonstrated significant oncolytic activity. These data also suggested that a clinically relevant chemosensitivity assay might enhance the selection of appropriate therapy.