The maximum phagocytic and bactericidal capabilities of normal human PMNs against a seroresistant strain of Pseudomonas aeruginosa were evaluated by morphological observation, uptake of radiolabeled bacteria, and quantitative killing methods. The number of bacteria killed per PMN increased from 3 to 23 as the bacteria-to-PMN ratio was increased from 3:1 to 100:1. Conversely, the percent of bacteria killed decreased from 94% to 3%. In monolayers, an increase in the inoculum from 10(5) to 10(8) cfu/ml was associated with greater phagocytosis and a sixfold increase in PMNs containing greater than or equal to 5 bacteria/PMN. With the use of 75Se-labeled P. aeruginosa, optimal phagocytosis was observed with 10 to 20 bacteria/PMN in 20% NPS. Maximum uptake of 70% occurred in 40 min. No difference was observed in the uptakes of live or heat-killed bacteria. The maximum number of bacteria ingested per PMN was 32 +/- 5 at the highest ratio tested (100:1). The use of altered opsonic sources indicated the need for the classical complement pathway for optimal phagocytosis. Thus study describes the requirements and necessary standardization parameters that were found to be essential for a highly reproducible method for measuring phagocytosis and killing of P. aeruginosa by normal human PMNs. This method could be employed for clinical assessment of partial opsonic or PMN dysfunction in the study of the interaction of PMNs and P. aeruginosa.