Expressions of p53 protein, a product of the tumor suppressor gene were studied in osteosarcomas relating to various prognostic factors. Thirty-four osteosarcomas were investigated immunohistochemically with a monoclonal antibody clone PAb240, which recognizes a common conformational epitope of mutant p53 proteins and another clone PAb1801, which reacts with both wild- and mutant-type p53 proteins. The results were compared with expressions of proliferating cell nuclear antigen (PCNA) and Ki-67 providing a simple method for the assessment of growth fractions of tumors. PAb240 stained nuclei and cytoplasm of tumor cells in 8 of 34 osteosarcomas (23.5%), whereas PAb1801 reacted in all 34 osteosarcomas (100%). Fifteen tumors (44.1%) showed positivity for PAb1801 in more than half of the tumor cells. Twelve patients were alive and thirteen were dead. Tumors from 9 patients (75%) who survived revealed only focal positive immunoreactions with PAb1801 and tumors from 6 patients (46.1%) who died revealed diffuse reactions. Twelve cases (35.3%) showed a high PCNA index (> 40%) and fibroblastic osteosarcomas revealed the highest PCNA positivity. Twenty-two cases (64.7%) revealed a very low Ki-67 index (less than 10%) and Ki-67 index showed a good correlation with PCNA positivity (r = 0.6247). Expressions of both wild-and mutant-type p53 protein, PCNA, and Ki-67 were not correlated with other clinical or pathological parameters.