Binding of the chemotactic peptide N-formyl-methionyl-L-leucyl-L-phenylalanine (FMLP) by neutrophils from neonates was measured to investigate whether an abnormality of this event that initiates chemotaxis could explain the decreased chemotaxis of neonatal neutrophils. No differences of FMLP binding by neonatal and control neutrophils were found at 4 and 22 degrees C when specific binding was assessed by analysis of binding isotherms and Scatchard plots. At 4 degrees C (n = 6) the average number of FMLP molecules bound per neutrophil was 13,854 (neonates) and 17,368 (controls); affinity expressed as Kd was 32 nM (neonates) and 34 nM (controls). At 22 degrees C (n = 5) the respective values were 22,587 FMLP molecules per neutrophil (neonates) and 21,082 (controls); Kd was 29 nM (neonates) and 23 nM (controls). Based on these data, we suggest that mechanisms responsible for decreased chemotaxis of neonatal neutrophils must involve events subsequent to initial FMLP-receptor interaction.