Serum from tolerant animals induced immunological enhancement of muscle allografts. Serum from both animals bearing tolerated skin allografts and animals in which tolerance to skin grafts disappeared was effective. No significant differences between these two types of allografts were found with respect to contractile and histochemical behaviour. Serum from animals in which skin allografts survived transiently following non-specific treatment induced weak immunological enhancement. The changes in contractile and histochemical properties of muscle allografts serve as adequate parameters for assessment of the success of immunological enhancement.