To investigate the effect of mercuric chloride on the pectoral muscel cells during chick embryogensis, chicken embryos were treated with mercuric chloride. Monoclonal antibodies against myosin were prepared for the localization of differentiation of thick filaments detected by the use of protein A-gold complex as a cytochemical marker. The effect of mercuric chloride was appeared not only to be ascribed with the reduced formation of myofibrilogenesis but also associated with induced change of morphology by the inhibition of protein synthesis.