ATP, when added to human polymorphonuclear neutrophils (PMNs) at concentrations similar to those attained extracellularly at sites of platelet thrombus formation (0.1 to 20 microM), causes an enhancement of N-formyl(methionyl)leucylphenylalanine (FMLP)-stimulated superoxide anion (O2-) generation. However, ATP by itself is an ineffective agonist for O2- generation by PMNs. The ATP-induced enhancement of O2- generation is associated with a shortened lag time in the response of PMNs to FMLP without a change in the median effective dose for FMLP, suggesting that signal transduction, rather than altered receptor affinity, is responsible for the enhanced oxidative response. Maximum enhancement of O2- generation is detected as early as 15 seconds and is maintained for at least 10 minutes. Of various nucleotides and nonhydrolyzable-ATP analogs test d, only ATP, UTP, and ITP were found to cause enhanced O2- generation by PMNs. Addition of ATP to quin2-loaded PMNs, in the absence of other stimuli, elicits a dramatic rise in [Ca2+]i which reaches a maximum of 500 to 800 nM at 30 seconds and slowly returns to baseline over 5 minutes. This ATP-induced rise in intracellular free Ca2+ concentration is correlated with the enhancement of FMLP-stimulated O2- generation both with respect to dose and nucleotide specificity. Stimulated Ca2+ uptake, rather than mobilization of intracellular Ca2+ stores, appears to be primarily responsible for the rise in intracellular free Ca2+ concentration. These studies indicate that an ATP-induced rise in intracellular free Ca2+ concentration, although insufficient by itself to elicit O2- generation by PMNs, is associated with a priming of PMNs for enhanced O2- generation when stimulated by other agonists.