The course of immunological parameters in a 2 year old patient with mediterranean kala-azar is reported. Plasma-fibronectin was reduced to 33% of the normal value at the time of diagnosis. When the clinical symptoms of the disease were most severe, a marked activation of complement without demonstrable circulating immune complexes was observed. Therapy with sodium-stibogluconate lead to improvement of the child's condition, normalization of complement reaction and fibronectin levels, and detection of circulating immune complexes. The activity of the disease seems to be well indicated by the level of C-reactive protein and the complement cleavage product C3 d. Incubation of living Leishmania donovani with normal human serum leads to activation of complement in vitro.