Twenty patients with prolonged septicemic salmonellosis (Group 1) and 20 with schistosomiasis mansoni (Group 2) were selected for this study. In both groups, the Widal test was done using antigens of the sample Ty 901 (S. typhi). The test was also applied in 6 group 1 patients with antigens prepared from salmonellae isolated from these patients (autoantigens). Titres over 1:200 were considered significant. Ten group 1 patients (50%) were positive for antigen "H" and 5 (25%) were positive for antigen "O". Three patients with negative "H" and "O" reactions became positive with high titres when using autoantigens. Two other cases maintained the same positive titres and one case showed a fourfold increase in titres when the test was done 'with antigens of the Salmonella isolated. The Widal test was positive in most patients infected with group D Salmonellae. Considering titres above 1:200, all cases were negative in Group 2. The authors conclude that the Widal test has low positivity in prolonged septicemic salmonellosis. The test may be valuable in the diagnosis of this disease when using S. paratyphi "A" and "B" antigens and a mixture of Salmonella antigens taken from other groups.