Infection of host cells with Leishmania, which are obligate parasites of mononuclear phagocytes, most probably involved chemotaxis of host cells towards the parasite. We have examined the chemotactic properties of a sonicate derived from L. mexicana amazonensis promastigotes for normal human peripheral blood monocytes and neutrophils. L. m. amazonensis sonicate exhibited chemotactic activity for monocytes and neutrophils. Treatment at 65 degrees C for 30 min, enhanced the activity for neutrophils but not for monocytes, while treatment at 100 degrees C for 60 min abolished the activity. Additional studies showed that the sonicate generated chemotactic activity in serum, presumably by activating the alternative complement pathway to produce C5a, for monocytes and neutrophils. Incubation of monocytes and neutrophils with the sonicate inhibited the chemotactic activity of these cells towards various chemoattractants. When the sonicate was heat-treated the inhibitory effect was lost, except when sonicate was used as a chemoattractant. These results indicate the presence of specific receptors for factor(s) from L. m. amazonensis promastigotes on human monocytes and neutrophils.