The pathway for inward Ca2+ movements in isolated human neutrophils was investigated. One-way 45Ca2+ influx into resting, steady-state cells amounted to approximately 6 mumol.l cell water-1.min-1. This uptake can be entirely accounted for on the basis of a carrier-mediated exchange of external Ca2+ for internal Na+. The counter transport exhibited trans stimulation of 45Ca2+ influx by internal Na+ (Km approximately equal to 26 mM) with a Hill coefficient of approximately equal to 2.6. There was substrate saturation by external Ca2+ (Km approximately equal to 0.15 mM) and by external Na+ (Km approximately equal to 30 mM); the two ions competed for the same binding sites. The exchange was noncompetitively inhibited by benzamil (Ki approximately 75 microM) and by some other amiloride analogues bearing a substituent on the terminal nitrogen atom of the guanidino group. Membrane depolarization enhanced the rate of 45Ca2+ entry, suggesting an electrogenic process; this voltage dependence was consistent with a coupling ratio of approximately 3 Na+:1 Ca2+. Hence, the Na+-Ca2+ exchange mechanism of neutrophils displays many features in common with those of other cell types. Studies in cells exposed to the tripeptide N-formylmethionyl-leucyl-phenylalanine (FMLP) indicate activation of the counter-transport system by the chemotactic factor. Thus, Na+-Ca2+ exchange may be at least partly responsible for the increase in 45Ca2+ influx and transient rise in intracellular free Ca2+ that are seen following stimulation.