The relationship between platelet surface negative charge and hyperfunction was examined by determining electrophoretic mobility (EPM), aggregability, and sialic acid of platelets in prostatic cancer, prostatic cancer with estrogen, prostatic cancer with estrogen and aspirin, prostatic hypertrophy, and healthy aged males. Estrogen treated prostatic cancer patients had significantly higher platelet EPM. A good linear correlation was found between sialic acid and EPM (r = 0.97, p less than 0.001). EPM was negatively correlated with primary aggregations by adrenaline and ADP but not with secondary or maximum aggregations, suggesting increased surface negative charge may inhibit primary aggregation. Estrogen and platelet population changes influenced surface negative charge. Neuraminidase removal of platelet surface sialic acid resulted in dose-dependent decreases of EPM which paralleled decreases in sialic acid. Aspirin treated patients and platelets incubated with aspirin in vitro both showed increased platelet EPM. These results suggest that platelet surface negative charge may directly affect platelet function.