LDL preparations obtained from normal individuals of known genetic type have been added to neutrophils from healthy donors. After incubation at 37 degrees C for 30 min, and washing in Hank's solution, in order to evaluate metabolic and bactericidal activities neutrophils were assayed by the nitroblue tetrazolium (NBT) quantitative test, using latex particles, and by E. coli killing test. LDL in aliquots of 100-500 microgram/2 x 10(6) neutrophils, inhibited NBT reduction and reduced neutrophil killing of E. coli by more than 50%. No difference was observed in the degree of inhibition using LDL and neutrophils obtained from donors of either the same or of different Ag phenotype. These data suggest that the immunoregulatory activity of LDL affects lymphocyte as well as neutrophil function.