A randomized double-blind study was performed to compare the side effects of long-term chemoprophylaxis of malaria with Fansidar (1 tablet a week) with those of a 300-mg weekly chloroquine regimen. This study was designed as a field trial with Austrian industrial workers in Nigeria and included 173 volunteers, 86 taking Fansidar and 87 taking chloroquine for 6 to 22 months. Only a few complaints were reported during that time, gastrointestinal disorders predominating in the Fansidar group and insomnia in the chloroquine group (3 cases each). The other complaints in both groups included one case each of skin rash and of visual disturbance, as well as one case of facial erythema after alcohol consumption in the Fansidar group and one of hair loss in the chloroquine group. Laboratory checks were performed at 3-monthly intervals, and included white and red cell counts, platelet counts and determination of GOT, GPT and alkaline phosphatase. There were no signs of drug-associated liver damage. In the Fansidar group there occurred a slight and transient decrease in the red cell count and in the chloroquine group a slight and transient decrease in the white cell count. Although statistically significant, these changes were without clinical significance. It is noteworthy that there were no cases of leucopenia in the Fansidar group. With the exception of one volunteer, who had discontinued his prophylactic drug regimen, malaria did not occur. Antibodies against blood stage parasites as determined by the indirect immunofluorescence test (IIFT), however, could be found at different stages of the study, which indicates that these two antimalarials are not causal prophylactic agents.