111Indium-bleomycin (111In-BLM) and 57Co-bleomycin (57Co-BLM) were prepared and their distributions were compared in the tissues, blood, and urine in tumor-bearing and in untreated mice and rats. Autoradiographs of electrophoresis gels showed that patterns for urine from untreated and tumor-bearing animals, collected 1-3 h or 48 h after injection of 111In-BLM were similar to those for in vitro mixtures of urine and 111In-BLM, but differed from the patterns obtained with 111InCl3 under in vivo or in vitro conditions. In rats bearing mammary adenocarcinoma, 48 h after administration of the radiopharmaceutical, the activity ratio of tumor to eleven different tissues was 1.2-4.6 times higher for injected 111In-BLM than for 111InCl3 (P less than or equal to 0.001 or P less than or equal to 0.05). Imaging with a gamma camera depicted tumors in mice more distinctly with 111In-BLM than with 111InCl3. These findings were interpreted as reflecting the stability of 111In-BLM in vivo. The tumor concentration (%dose/g) was higher for the viable area than for the necrotic area for 111In-BLM, but the reverse was true for 57Co-BLM.