In the course of Onchocerca volvulus infection in subjects with from light to heavy microfilarial count in skin snips occurred important alterations concerning both humoral and cell-mediated immunity. Serum examination for immunoglobulins showed high synthesis of IgG, IgM and IgE. The level of IgM was found to be correlated to the intensity of the infection, an important reduction of T4 cells was observed and is associated with hyporeactivity to in vitro stimulation to phytohemagglutinin A (PHA). The considerable loss seen in T4 cells function could lead to a reducing of cell-mediated immune mechanisms and humoral response needed in the acquisition of resistance.