The effect of opsonization of zymosan with serum on the repiratory burst of haemocytes isolated from the Pacific oyster Crassostrea gigas was investigated. The reactive oxygen species (ROS) produced by the respiratory burst of haemocytes in response to the opsonized or unopsonized zymosan were measured using chemiluminescence (CL). The CL values were increased according to the increment of haemocyte number. The degree of CL amplification by increase of haemocytes from $0.1{ imes}10^6$ to $0.5{ imes}10^6$ cells/ml was 3-5 times, but comparatively low amplification was elicited by increase of haemocytes from $0.5{ imes}10^6$ to $1{ imes}10^6$ cells/ml. Chemiluminescences produced by the haemocytes in response to the zymosan opsonized with oyster serum were considerably higher than the CL produced by the haemocytes in response to the unopsonized zymosan.