The exposure of the blue mussel Mytilus trossulus to increased concentration of cadmium in seawater resulted in the synthesis of a new protein of the approximate molecular weight of 60 kDa. This protein is immunologically crossreactive with the 60 kDa heat shock protein of the moth Heliothis virescens. The latter one is a member of a highly conserved family of proteins found in animals at various levels of organization. This result indicates that changes in the abundance of stress proteins may prove useful as a biomarket of mussels exposure to specific toxicants.