The capacity of various leucoattractants to enhance, or unfold, receptors for complement (C3b) on human blood monocytes has been studied. A number of recognized monocyte chemoattractants including casein, supernatants from C. parvum 10390 and from human lymphocytes (cultured either in the presence or absence of phytohaemagglutinin) and the formyl-methionyl peptides, F-Met-Leu-Phe, F-Met-Met-Phe and F-Met-Phe, increased the percentage of monocytes which formed rosettes with IgM-sensitized sheep erythrocytes coated with complement. Comparable results were achieved irrespective of whether purified components of whole serum was used as a source of complement. In contrast, there was no significant increase in EAG (Fc) rosettes with those doses of casein which gave enhancement of C3b receptors. A small degree of complement receptor enhancement was observed with histamine but the unformylated peptides, Met-Leu-Phe and Met-Met-Phe, were without apparent effect. Maximal receptor enhancement was obtained at 30 min but when the leucoattractant was removed, enhancement was reversible, returning to normal values in approximately 120 min. Monocyte complement receptor enhancement increased with temperatures between 0 degrees C and 37 degrees C. These data (1) confirm and extend our previous findings on leucoattractant-induced enhancement of complement receptors on human monocytes; (2) indicate that the phenomenon may have potential as a clinical test for monocyte function both in health and disease.