Chemiluminescence was used to determine opsonic requirements for interaction of Cryptococcus neoformans with human peripheral blood polymorphonuclear leukocytes (PMNLs) and monocytes (MNs). Peak chemiluminescence (mean +/- SD) was 172 +/- 25 X 10(3) for PMNLs and 50 +/- 13 X 10(3) for MNs with 20% pooled normal human serum vs. 10 +/- 3 X 10(3) for PMNLs and 6 +/- 1 X 10(3) for MNs without serum (P less than 0.005 for each comparison). The nature of the serum requirement was investigated; alternative pathway serum complements were necessary. The alternative pathway was activated directly by C. neoformans without the need for specific antibody. Chemiluminescence provided a quantitative and reproducible assay of the interaction of human phagocytes with C. neoformans and may be useful in studying opsonic requirements of other fungi.