We have systematically analyzed the proliferative properties of the clonogenic cells of the R3327 transplantable rat prostate adenocarcinoma (colony forming units, prostatic adenocarcinoma, CFU-PA). Pronase was determined to be more effective than either trypsin or collagenase in obtaining the largest number of viable cells from a solid R3327 tumor. Digestion with 2.5 mg/ml yielded a maximum of 5.6×104 CFU-PA/g of tumor tissue, with higher concentrations resulting in a substantially lower fraction of CFU-PA while producing larger overall cell yields. Bacto-agar at 0.35% supported the growth of the largest number of CFU-PA and was sharply concentration dependent with concentrations greater than 0.4% completely suppressing CFU-PA growth. The addition of conditioned medium (CM) from R3327 liquid cell cultures to agar cultures resulted in a specific twofold increase in CFU-PA/104 cells, whereas CM from R3327A cells was less effective and CM from rat skin fibroblasts least stimulatory. The addition of washed rat red blood cells (RBC) either within the agar culture itself or in an overlayer was highly stimulatory, resulting in as much as a fivefold increase in CFU-PA to 6 to 8×104/g.