A micro-ELISA technique has been developed for the diagnosis of onchocerciasis. With the use of a low molecular weight (LMW) fraction of a soluble, adult Onchocerca volvulus extract as an antigen, the problem of nonspecificity observed with the crude worm extract is negligible. Seventy-three sera from proven cases of other filarial infections were weakly reactive or nonreactive. From cross-sectional and longitudinal surveys in a West African village hyperendemic for onchocerciasis, we concluded that this assay detects antibodies elicited when female worms start to produce microfilariae. Thirty-two of 33 children aged less than or equal to 15 years with positive skin snips were seropositive. High antibody levels could also be found in children who only became parasitologically positive 1-4 years after seroconversion. This leads to the conclusion that serology is more sensitive than the 2-snip method. In individuals with chronic infection (exposed for greater than 15 years), antibody titers were consistently lower (independent of the density of skin microfilariae) than in the early phase of a patent infection. A comparative serological study in a mesoendemic village revealed that the assay can serve to assess onchocerciasis prevalence by testing individuals aged 5-15 years.