It has been proven that the abnormal activation of ras oncogene and the expression of its product P21, a 21,000-dalton molecular weight protein, are present in a number of human tumors. Using an ABC immunoencymatic staining technique and a P21-specific monoclonal antibody, SCI-Oncogema I, we have detected the expression of P21 in formalin-fixed, paraffin-embedded samples from 53 bladder tumors and 7 normal bladder mucosae. The chi 2 test was used for data analysis. The results demonstrate that P21 is only expressed on tumor tissues. It is also seen that the percentage of P21 positivity increases, obviously in parallel with the increase of pathological grade and stage. The morbidity rate due to tumor recurrence in 32 P21-positive patients was only 9% (3/32), whereas that in 21 P21-negative cases was 67% (14/21). This suggests that in patients with bladder tumors P21 positivity could be considered as an objective indicator of a favorable prognosis.