Tumor factors predicting for prognosis in metastatic breast cancer. The presence of P24 predicts for response to treatment and duration of survival. Fifty-one patients with metastatic breast cancer were investigated to determine tumor parameters with prognostic significance. Investigations included determinations of P24 content by immunocytochemical means using a monoclonal antibody to P24 protein; immunocytochemical analysis of estrogen and progesterone receptors; ploidy analysis by flow cytometry, and histologic grading. There were significant correlations between the presence of P24 and estrogen receptor, between histologic grade and P24 expression, and between estrogen and progesterone receptors. Of the tumor factors investigated only P24 protein was, however, of prognostic significance. Patients with P24-positive tumors had a significantly higher rate of response to treatment as well as more prolonged duration of response and duration of survival from diagnosis of metastatic disease. None of the other variables investigated were significantly predictive of outcome. P24 protein may be a useful predictor of prognosis in metastatic breast cancer. 