Human blood monocytes comprise two subpopulations: one migrates to the chemoattractant, N-formylmethionyl-leucyl-phenylalanine (fMet-Leu-Phe), and has saturable binding sites for this peptide; the other does not migrate and exhibits little peptide binding. To determine if expression of binding sites was a function of monocyte maturation, we depleted human subjects of blood monocytes by leukapheresis so that the circulation was repopulated by monocytes released from the bone marrow. Pre- and postleukapheresis monocytes were then compared for fMet-Leu- [3H]Phe binding, superoxide generation, and chemotactic responses. No significant differences in peptide binding curves were found, suggesting that receptor expression was stable over the maturational span represented by these two groups of cells. This supports the hypothesis that there are two distinct lineages of monocytes with respect to expression of receptors for fMet-Leu-Phe. An additional finding of interest was that the number of chemotactically responsive cells immediately postleukapheresis was half the control. This was a transient state; monocyte responses were normal 3 hr after termination of leukapheresis, suggesting that they rapidly become functionally mature.