In order to assess the possible suppression of immune responses to non-specific antigens in patients with generalized onchocerciasis, a vaccination study was conducted among the population of eight Liberian villages. Hundred and eighty-four persons infected with Onchocerca volvulus and 142 non-infected individuals were tested for delayed cutaneous hypersensitivity to tetanus antigen using the Multitest Mérieux. Prevalence of malaria and infections with intestinal nematodes with larval migration to the lung were much the same in all the villages examined. Other parasitic diseases had no significant influence. Persons aged 2-29 years with onchocerciasis had a significantly lower prevalence of positive tetanus reactions (p less than 0.05). Hundred and eighty persons showing a negative skin test were vaccinated twice with 75 i.u. tetanus toxoid and cell-mediated as well as humoral immune responses were recorded three months later. Analysis of sera collected before vaccinations indicated that 56% of the vaccinees had tetanus antibody levels of 0.01 i.u./ml or more and had, therefore, received a booster vaccination. Rates of seroconversion or booster were the same in persons with or without onchocerciasis (81% and 82% respectively). The conversion rate in the delayed cutaneous hypersensitivity test, however, was significantly lower in those infected with O. volvulus (67%) as compared to the controls (92%, p less than 0.001). These results demonstrate a diminished cell-mediated immune response to tetanus toxoid vaccinations in patients with generalized onchocerciasis. Considering the unimpaired humoral immune response, no changes in vaccination schedules are recommended.