M onocytes, macrophages, and neutrophils will demonstrate several important cellular functions in response to synthetic formylated oligopeptides. N-formyl-norleucyl-leucyl-phenylalanyl-norleucyl-tyrosyl-lysine (fNLPNTL) was a potent chemoattractant for human blood monocytes; a 1.0-nM concentration induced a maximal chemotactic response. Binding of 125I-labeled fNLPNTL to the monocyte formyl peptide receptor was rapid, specific, and saturable at 4, 24, or 37 degrees C. At 4 degrees C, monocytes from several different donors demonstrated between 10,000 and 18,000 receptors/cell with a dissociation constant (Kd) of 1.7-2.7 nM. The association of the 125I peptide with the cells was irreversible at the elevated temperatures and exceeded the amount of surface receptor by approximately four-fold, suggesting receptor-mediated peptide endocytosis. Processing of rhodamine-labeled fNLPNTL by monocytes was observed directly by video intensification microscopy. At 37 degrees C, diffuse membrane fluorescence was seen initially, followed by rapid aggregation and internalization of the peptide. Monocytes incubated with fNLPNTL displayed a temperature dependent loss of surface binding capacity (receptor down-regulation). This decrease was due to a decrease in surface receptor number rather than to a decrease in receptor affinity. A dose-response curve for peptide-induced receptor down-regulation correlated with a dose-response curve for 125I-labeled fNLPNTL uptake, suggesting that each uptake event led to the loss of one surface receptor. Surface receptor replenishment following down-regulation was rapid and not dependent on new protein synthesis, but was inversely related to both the time and peptide concentration used to induce down-regulation. An exact correlation between receptor down-regulation and functional deactivation of the chemotactic response could not be demonstrated.