The host response to Plasmodia includes the production of enlarged populations of peripheral blood monocytes and tissue macrophages in the spleen and the liver. Since the hyperplasia of the mononuclear phagocyte system is believed to arise as a consequence of an enhanced blood monocyte influx, we tested monocyte chemotactic responsiveness in 19 patients with acute primary attack malaria. In addition, the neutrophil chemotaxis was measured in 12 patients. Before the initiation of antimalarial treatment a significant depression of monocyte chemotaxis was observed in approximately half of the patients when compared with healthy control subjects. The depression was found in Plasmodium falciparum malaria as well as in P. vivax or P. ovale malaria patients. The defective responsiveness was not receptor specific, since the responses towards casein and zymosan activated serum proved to be equally suppressed. The monocyte chemotaxis was followed in 14 of the patients, during treatment and after complete recovery. After 3 days of treatment the response had improved in most of the patients, and after 7 days all patients had a normal monocyte chemotaxis, which remained normal after one month. No significant differences between P. falciparum and P. vivax/ovale malaria was observed with respect to blood monocyte chemotactic responsiveness. Neutrophil chemotaxis in patients with P. falciparum infections was similarly suppressed before treatment (54% of controls), was still defective after 3 days of treatment, and nearly normalized after 7 days (87% of controls). Furthermore, monocyte phagocytic and candidacidal activities were assessed in the same patients on admission and during the follow-up. In contrast to chemotaxis, these functions were normal in all of the patients whenever measured. In conclusion, not all cell functions were altered in concert, and the previously unreported suppression of chemotactic migration might reflect a change in blood leucocyte subpopulations, deactivation in vivo or a direct suppressive effect of plasmodia induced products.