To investigate possible immunological consequences after arterial infusion of microencapsulated anticancer drug (chemoembolization), cellular immune parameters in peripheral blood were monitored for 4 weeks after chemoembolization in 20 patients with urological malignancies including 7 renal cell carcinomas, 2 renal pelvic carcinomas, 8 bladder carcinomas and 3 prostatic carcinomas. Natural killer (NK) cell activity and OKT4/8 ratio were augmented in 67% and 83% of the cases after chemoembolization respectively. Mature NK cell population (Leu7-OKT 16+) in these cases also showed a trend of continuous increase for more than 4 weeks. On the other hand, a small number of the patients showed an augmentation of OKT3+ cells, Ia-1+ cells or response to phytohemagglutinin. Histopathological examination of regional lymph nodes in 9 patients who underwent chemoembolization for renal cell carcinoma showed stimulating reactions in 8, while lymphocyte depletion in 1. These findings suggest that inhibition of micrometastasis by the enhanced natural killer cell activity could be anticipated in more than half of cases after chemoembolization.