The adherence and cytotoxicity of granulocytes to microfilariae of Onchocerca volvulus were examined in vitro. Reactivity and modulation by diethylcarbamazine of isolated eosinophilic and neutrophilic granulocytes from patients with generalized and chronic hyper-reactive onchocerciasis (sowda or localized form) from endemic foci in Liberia were evaluated under varying serum conditions. In the presence of pooled sera from patients with generalized onchocerciasis granulocytes from both polar groups of patients exhibited similar adherence rates, whereas immobilization rates were higher for eosinophils than for neutrophils. In localized onchocerciasis, the use of autologous serum resulted in a significant decrease in adherence and immobilization rates for both eosinophils and neutrophils. After preincubation of eosinophils, but not of microfilariae, with diethylcarbamazine autologous serum-mediated adherence and cytotoxicity were enhanced to rates similar to those found with pooled serum from individuals with generalized onchocerciasis. These results suggest that granulocytes from both forms of onchocerciasis did not differ with respect to their anti-parasitic reactivity and that antibodies as well as additional serum factors appear to contribute to the functional activity of these effector cells. The findings indicate that predominantly eosinophils, compared to neutrophils, damage the larvae of O. volvulus and support earlier observations which suggest that diethylcarbamazine influences the effector cells rather than the parasite itself.