In seven Liberian villages of different endemicity for onchocerciasis 629 persons were tested for tuberculin reactions using 5 i.u. purified protein derivative or old tuberculin. No correlation between infection with Onchocerca volvulus and reactivity to tuberculin was detected on a community level, but a direct comparison of results of infected and non-infected individuals in two neighboring villages revealed a lower responsiveness in persons with onchocerciasis. The conversion rate three months after BCG vaccination was significantly lower in children with onchocerciasis (48%) than in controls (85%). The possible impact of this finding on the efficacy of BCG vaccinations is discussed. Girls aged 2-16 years were screened for rubella antibodies using a latex agglutination test. Sera from 74 girls, who showed no antibodies, were examined before and four months after rubella vaccination by haemagglutination-inhibition-test and haemolysis-in-gel-test. No difference in the humoral immune response of girls with or without onchocerciasis was found. Seroconversion rates were 98% and 100%. Delayed cutaneous hypersensitivity (DCH) to seven ubiquitous recall antigens was tested in 147 persons infected with O. volvulus and compared to the results of 142 non-infected controls. A reduced responsiveness was seen in persons with onchocerciasis, the difference being greatest in young children 2-7 years of age. In connection with the results of tetanus vaccinations reported earlier, these findings verify the existence of a reduced in vivo responsiveness of cell-mediated immunity to non-specific antigens in onchocerciasis in the rain forest of West Africa. Humoral immune responses however appeared to be unaffected.