The use of chemiluminescence techniques t o study the interaction between bacteria and phagocytes has been useful for examining the extent t o which serum factors, such as opsonins, are important in internalization of the organisms and the response of the cell t o phagocytosed bacteria. However, such methods have been limited by the number of experiments which can be performed at one time using most commercial luminometers. However, the recent introduction of the Amerlite microtitre plate luminometer allows the measurement of chemiluminescence responses in %-well microtitre plates. Using this instrument, lucigenin-enhanced chemiluminescence can be detected from as few as 5000 cells (polymorphonuclear leukocytes or monocytes) per well with a 1 :I0 ratio of cells t o zymosan particles opsonired with 10% serum. The opsonic capacity of up t o 100 sera can be measured in triplicate wells in a single experiment using four microtitre plates and polymorphonuclear leukocytes prepared from less than 40 ml freshly obtained venous blood. We are currently using this technique to investigate the effect of serum opsonins on the interaction between normal human polyrnorphonuclear leukocytes and monocytes with rnycobacteria of three species (Mycobacteriurn leprae, M. tuberculosis, and M. aviurnintracellulare). Other possible applications of this method are discussed.