P53 protein expression in malignant cells of five patients with Burkitt lymphoma (BL) and from two patients with B-cell acute lymphoblastic leukemia (B-ALL) was examined with anti p53 protein monoclonal antibodies PAb1801, PAb240 and p53-D07 using an immunocytochemical technique. Four of the seven patients were positive. The distribution of positive staining within the cell was predominantly in the nucleus. The reactivity of PAb240 was weaker than that of the other antibodies. In addition, three of the four positive cases showed the same abnormal karyotype; translocation (8;14) (q24;q32). All of the four positive cases died due to relapse of their primary disease. The three negative cases did not show karyotypic abnormalities and are still alive and well. In conclusion, p53 immunostaining technique may be useful for predicting the clinical outcome of B-cell malignancy.