In an attempt to evaluate the degree of in vitro cellular sensitization to tumor and its relationship to clinical responsiveness, direct leukocyte migration tests were carried out in patients with varying degrees of adenocarcinoma of the prostate employing pooled allogeneic extracts of normal, benign, and malignant prostatic tissue as a source of antigen. Cell-mediated immunity to presumably common prostatic tumor associated antigens was observed. The degree of sensitization of clinically significant specific reactivity of the patients' leukocytes to malignant prostatic tissue was greatest in patients with localized disease, low-grade tumor, and clinically inactive disease than in patients with advanced disease, high-grade tumor, and clinically active disease. Evaluation of the possible correlation of specific reactivity to malignant prostatic tissue as a prognostic index of clinical responsiveness revealed a positive correlation with the degree of sensitization in 3 (43 per cent) of 7 patients. Correlation in 4 patients was questionable because of observations of "stimulation" of migration rather than inhibition, suggested by some to be reflective of weak sensitization to tumor. Evaluation of a larger patient population as well as a prospective study of the relationship of the degree of sensitization and clinical responsiveness will be necessary before any definitive conclusions may be drawn regarding the present observations.