A radiolabeling method was developed to investigate the interaction in vitro between Cryptococcus neoformans and Lewis rat alveolar macrophages (AM phi). AM phi were harvested by lung lavage, monolayers of adherent cells were established in wells of microtiter plates and [51Cr]-labeled yeast cells were added to the monolayers. After removal of extracellular yeasts, the adherent radioactivity associated with the AM phi was directly proportional both to the number of yeasts added and to the number of yeasts per AM phi as determined by microscopic examination of Giemsa-stained monolayers. Phagocytosis (attachment and/or ingestion) of radiolabeled C. neoformans by AM phi was a sensitive, quantitative and reproducible assay for the evaluation of the AM phi-C. neoformans interaction. AM phi were able to phagocytose encapsulated strains of C. neoformans. The extent of phagocytosis was inversely related to the capsule size. Normal rat serum (NRS) was an excellent source of opsonins for the ingestion. Inactivation of serum complement or depletion of C3 by affinity chromatography removed most of the opsonic activity of NRS. Specific antibodies against C. neoformans did not increase phagocytosis.