The authors recorded clinical histories and tested serum for the presence of malaria fluorescent antibodies in 160 healthy Europeans who had been living for more than 4 weeks in West or Central Africa. Malaria or fever of unknown origin occurred in 37 of 50 subjects who were careless about taking prophylactic drugs while abroad. Out of 110 people regularly taking suppressive amino-4-quinoline therapy, 21 had presented febrile attacks but serological tests were only positive in 8 cases. Positive serological reactions at low titers were obtained in 3 subjects with no history of past infection and who had faithfully taken suppressive medications. These results confirm the value of the malaria immunofluorescence test for the detection of occult malaria in blood donors outside endemic areas, and explain the necessity to consider previous regular, irregular or absent chemoprophylaxis before interpreting the serological results of a febrile patient returning from overseas.