An ELISA was devised to quantitate the number of group B streptococci (GBS) adherent to flat-bottomed 96-well microtiter plates. Ingestion of GBS by bovine polymorphonuclears (PMN) led to a decrease in ELISA values. This ELISA quenching proved to be proportional to phagocytosis. The method is suitable for kinetic studies of phagocytosis using glutaraldehyde to block the uptake of bacteria. The ELISA quenching test is applicable to a large number of samples and permits the use of the semi-automated equipment developed for ELISA.