The chromogenic substrate p-nitrophenylphosphorylcholine was used to measure phospholipase C (heat-labile hemolysin) production by clinical strains of Pseudomonas aeruginosa. Analysis of strains from various types of infection showed that phospholipase C production by urinary tract isolates was significantly greater than that by lung, blood, or other isolates (e.g., wounds, etc.). The above method was also found to be effective for the isolation of several classes of phospholipase C mutants.