The Rift Valley fever epidemic, which arose in the south of Mauritania beginning on October 15, 1987, enabled a comparative study of different diagnostic methods among humans. During the first two weeks of the epidemic, four parallel methods were used: inoculation into Aedes pseudoscutellaris cells, inoculation intracerebrally into suckling mice, tests by immunocapture of the circulating antigen and detection of type IgM gammaglobulins. Of 370 examined sera, 181 showed at least one marker of recent infection. Inoculation into A. pseudoscutellaris cells was by far the most sensitive and easiest method to use. Detection of the antigen by immunocapture was also a useful technique, since it allowed quick aetiological diagnosis or examination of sera conserved under poor conditions. However, its sensitivity was weak, as it could only detect 26% of positive cases. Vero cells used on a limited scale, in this particular case seemed less sensitive than A. pseudoscutellaris cells. Of a total of 991 sera, 221 diagnoses were reported by discovery of the virus and 271 by detection of specific IgM. In every case, A. pseudoscutellaris cells seemed most appropriate as the system of reference.