Polymorphonuclear cell function tests were performed in 340 patients with recurrent unexplained infections. Serum-dependent defects in phagocytosis and intracellular killing of Candida albicans (Candida opsonin and procidin defects) were specifically associated with susceptibility to recurrent pyogenic infections, Candida opsonin defects occurring in 12.7% and Candida procidin defects in 20.8% of patients compared with 3.7% of healthy controls. Saccharomyces cerevisiae opsonin deficiencies were also found frequently in patients with recurrent infections, being present in 18.2% compared with 7.4% controls, although not specifically in those with pyogenic infection.