Microbicidal phagocytic function and oxidative metabolism of the hemocytes of 2 oyster species, Ostrea edulis and Crassostrea gigas, were investigated using a luminol-enhanced chemiluminescence (technique. First, experimental parameters adapted to marine bivalve hemocytes were established on hemolymph pools of C. gigas in order to obtain qualitatively and quantitatively homogeneous samples and so to perform statistically comparable assays. The use of Modified Alsever Solution (allowed hemocytes to be kept non-aggregated and in a non-stimulated state until starting the assays. A number of aliquots of 2 X 10(5) hemocytes with an MAS final concentration of 3.5 % showed high chemiluminescent response after stimulation by zymosan particles A particle:hemocyte ratio of 80:1 gave optimal CL activity. Activity was inhibited by cytochalasin B, a phagocytic inhibitor, and by sodium azide, the latter indicating the involvement of oxygen products. Having thus defined suitable parameter values, the CL protocol was then applied to 0. edulis for qualitative and quantitative analyses of respiratory burst capacity at the species and individual levels. Results suggested inter- and intra-specific variability of CL responses. Under the same experimental conditions, 0. edulis hemocytes generally displayed higher CL activities than C. gigas. Moreover, clear individual variability was demonstrated. The same experimental number of hemocytes showed great differences in CL responses between individuals suggesting a possible correlation with hemogram characteristics such as cell type percentages.