Clinical efficacy of a new preparation of peplomycin emulsion in hydroxypropylcellulosum (HPC-PEP) was studied in 26 patients to compared with that in 14 patients administered with 60 mg of PEP in 20 ml saline (S-PEP). The HPC-PEP was a mixture of 90 mg PEP suspended in 30 ml of 1% HPC. Both of preparations were retained in the bladder cavity over an hour after the instillation. Intravesical instillation was performed once for the patients with HPC-PEP, and 10 times repeatedly for the patients with S-PEP. A clinical evaluation was made on the basis of cytoscopic finding and cytology one week after the final instillation. According to the degree of tumor reduction, the results were classified into "disappearance", "greater than 50% reduction," and "no alteration or further growth" of primary tumor, which were referred respectively to "complete response (CR)", "partial response (PR)" and "not changed (NC)". The rates of CR and response were 27 and 73% respectively for the HPC-PEP administered patients, which were significantly higher than those of 8 and 43% respectively for the patients with S-PEP. In terms of configuration and number of tumor, intravesical HPC-PEP treatment was found to be superior to intravesical S-PEP treatment. In HPC-PEP treatment, a series of untoward symptoms such as bladder irritability and leukopenia was encountered at a frequency of 8%, which is much less than those seen in the S-PEP treatment. These clinical date suggest that HPC-PEP treatment against the superficial bladder tumors is superior to conventional S-PEP instillation in terms of administration frequency and potent doses available to tumor reduction.