Candida albicans cells which survive ingestion and multiply within phagocytes develop characteristic filamentous pseudogerm tubes. Candida cells killed by phagocytic leukocytes develop prominent alterations in Giemsa-staining characteristics; this reflects degradation of cyanophilic cytoplasmic components, probably ribonucleic acids. The numbers of these partially degraded organisms, termed “ghosts,” correlate closely with the percentage of Candida determined by an independent method to be nonviable. An assay, which makes use of these changes in morphological and staining characteristics of ingested C. albicans, was developed to evaluate the candidacidal activity of peripheral blood phagocytes. Neither myeloperoxidase-deficient neutrophils nor those from patients with chronic granulomatous disease killed C. albicans effectively, confirming observations made previously. Whereas myeloperoxidase-deficient cells were able to retard the intracellular germination of C. albicans, neutrophils from patients with chronic granulomatous disease lacked this ability. The candidacidal activity of monocytes and eosinophils in small samples of peripheral blood can also be measured by the new assay.