An in vitro double soft agar technique was used to culture 91 human urologic tumors including 37 renal cell, 40 uroepithelial, 7 prostatic and 7 testicular cancers. Cells from 31 of 37 renal, 32 of 40 uroepithelial, 3 of 7 prostatic and 4 of 7 testicular cancer specimens grew to the extent that they could be used in chemosensitivity testing in soft agar (greater than or equal to 30 colonies per control plate). With this assay system, a very high growth rate (70/91; 77%) was obtained. The in vitro response rates of greater than or equal to 10% were noted with mitomycin C, 5-fluorouracil, cisplatin, bleomycin and vincristine in renal cell cancers, and with vincristine and cisplatin in uroepithelial cancers. Drug sensitivity studies showed that the rates of in vitro sensitivity of uroepithelial cancers were close to those obtained in general clinical experience, while the rates of the vitro sensitivity of renal cell cancers were considerably higher than the rates found clinically. It is concluded from this study that in vitro chemosensitivity testing by clonogenic assay is likely to be a useful tool in the treatment of urologic cancers, but that a simple definition of sensitivity cannot be applied for all types of tumors.