The in vitro effects of pyrene (polycyclic aromatic hydrocarbon) on haemocyte and haemolymphatic parameters of the Mediterranean clam, Ruditapes decussatus, were tested using two concentrations (10^-3 and 10^-5 mg mL^-1). After 24 h contamination period, pyrene effects were monitored on individual haemolymph samples. The toxic exposure caused significant alteration in the immune parameters of clams. It increases the antibacterial activity, the total haemocyte count and the haemocyte mortality. The increase was associated with the decrease in lysozyme and esterase activities which reflects a decrease in secretion in order to reduce energy expenditure. Modulation of immune parameters in the Mediterranean clam by pyrene exposer suggested that PAH pollution may be related to enhanced susceptibility to diseases.