We describe a new quantitative fluorimetric assay for phagocytosis of bacteria. A suspension of fluorescein-labelled bacteria (Micrococcus lysodeikticus) is mixed and incubated with phagocytes. After fixation with paraformaldehyde, the excess non-phagocytosed and adsorbed bacteria are lysed with a solution of lysozyme in phosphate-buffered saline. After washing of the phagocytes, the fluorescence of those that have ingested the labelled bacteria is measured with a spectrofluorimeter. We report results obtained with different types of phagocytes which show that this method allows sensitivity, saturation and kinetic studies and the calculation of a phagocytic index.