Ranimustine (MCNU) has been shown to exhibit high antitumor activity and broad antitumor spectra against various experimental tumors. These effects were comparable to those of nimustine (ACNU). However, clinical applications of ACNU are indicated to various types of malignancies including solid tumors, while those of MCNU are almost limited to hematological ones. Therefore, the antitumor activity of MCNU was examined against 55 specimens from 15 types of solid tumors and compared with those of ACNU and 8 other drugs. Drug sensitivity was examined by a morphological method measuring the proportion of degenerative changes in the nucleus of drug-treated and untreated tumor cells. MCNU showed antitumor activities (measured by karyorrhexis) against adenocarcinoma of the lung, squamous cell carcinoma of the lung, renal cell carcinoma, bladder tumor, ovarian cancer and brain tumor. In addition, MCNU and ACNU showed a similar positive rate (15-16%) in this experiment and this was the highest among all drugs examined. Although MCNU and ACNU showed similar antitumor spectra, a clear difference was observed when the antitumor activities of both drugs were compared in each identical specimen. These results clearly suggest that MCNU is worthy of clinical study to examine the antitumor activity against various solid tumors.