The aim of this investigation was to quantify the effects of tributyltin (TBT) on the immune reactivity of haemocytes from the cultivated clam Tapes philippinarum (Adams and Reeve, 1850) using a series of in vitro bioassays. It is known that TBT has adverse effects on cellular immune functions like mobility, phagocytosis and lysosomal enzyme activity. As defining TBT-sensitive immunologic biomarkers in sentinel organisms is important in the field of ecotoxicology, the authors propose three indexes, amoebocytic (A.I.), phagocytic (P.I.), and lysosomal activity (L.A.I.), as sensitive and useful biomarkers to assess environmental risks due to TBT contamination.