The biological signs of typhoid fever are studied on the basis of 90 cases collected over a period of 5 years. Average neutrophil granulocyte count before treatment was 5000 with extremes ranging from 1400 to 12180. The development of granulocytopaenia during treatment may be seen not only in patients treated with phenicols but also in those treated with ampicillin or the combination trimethoprim-sulphamethoxazole. It was possible to isolate the organism in 88 per cent of cases, either by blood culture (79%) or by stool culture (23%). Antibodies (O and H agglutinins) were found in only 86 per cent of cases, and for H agglutinins only there was a significant and transient increase in antibodies. This underlines the importance of the combined examination of three biological criteria - blood culture, stool culture and serology - in reaching the diagnosis of typhoid, the relatively asymptomatic forms of which are becoming increasingly frequent.