Estramustine phosphate (EMP) influence on human monocyte phagocytosis of fluorescein isothiocyanate (FITC)-labeled yeast cells was measured in vitro. The method used, a modification of Hed's technique (FEMS Microbiol Lett 1:357, 1977), can differentiate between yeast cell engulfment and adherence to the phagocytotic cell surface. EMP is now accepted in the treatment of advanced prostatic carcinoma. In concentrations corresponding to the clinical situation (20-40 micrograms/ml), it dramatically inhibited the process of phagocytosis. The engulfment phase was inhibited, whereas cell adherence was less affected. This might be due to direct interaction with the microtubule system. The effects were totally reversible. In contrast, the metabolites estradiol and normustine did not affect engulfment of yeast cells, either as single agents or combined. The results demonstrate that the EMP complex caused an impaired phagocytosis, which could be of pathophysiological significance in the compromised cancer patients.