Luminol-dependent chemiluminescence produced by peripheral blood phagocytic cells was measured using a light detecting instrument, the Luminometer 1250 (LKB Wallac). Although less sensitive than liquid scintillation counters, the Luminometer can measure chemiluminescence effectively and detect experimentally induced and clinical variations using small numbers of cells. It is small, inexpensive, simple to use and allows temperature within the reaction vial to be controlled accurately. Light emission can be recorded either graphically or by an intergrated digital printout. Luminol-dependent chemiluminescence is influenced by the presence of contaminating red blood cells and by composition of the medium, particularly presence of phenol red. HEPES buffer and foetal calf serum. Whereas clinical comparisons of opsonic activity are easily performed, comparisons of cellular activity require a high and uniform standard of cell preparation, characterization and maintenance.