During an epizootic of Rift Valley fever in South Africa in 1974/1975, mainly affecting sheep and cattle, a large number of human cases occurred. The majority of the patients, mostly farmers, farm labourers and veterinary surgeons, acquired the infection while handling the carcasses of animals which had died of Rift Valley fever. Some gave no history of contact with infected animals and it is presumed that they were infected by mosquitoes. Complications were common. Retinitis clinically associated with defective vision occurred in about 20% of patients investigated. In others, meningo-encephalitis developed; 1 patient died and the brain showed perivascular cuffing and round-cell infiltration. In 110 cases the diagnosis was confirmed in the laboratory, in 17 by isolation of the virus and in 93 by the finding of an antibody response in serological tests. In 3 fatal cases the virus was isolated from the liver, and in a further 4 fatal cases in which virus isolation was not attempted, there was clinical and epidemiological evidence of the diagnosis; in each case a haemorrhagic state associated with hepatitis had developed. In 1978, follow-up studies showed that immune rates among residents on affected farms varied from 10% in children to 17,1% in adult males, with an overall immunity rate of 14,5%.