The Brachy (T) gene was introduced by 2 or more backcrosses into the Bagg albino, dba, SK, la, 19a and Mus bactrianus strains. In these strains the effect of the T gene in shortening the tail was decreased as compared with its effect in the original Z Brachy stock; in 2 of them (la and Mus bactrianus) normal overlaps occurred indicating the presence of a factor or factors capable of suppressing the gene in some of the offspring. Introduction of the D-short (Sd) gone into the Bagg albino strain resulted in the opposite effect, namely, an increased shortening of the tail. By selection of heterozygous Brachy mice, short tail and long tail Brachy lines were developed. The modifying genes that affect the tail length of Tt mice have little if any effect on the time of death of TT mice. The bearing of these ob-servations on gene interaction and on Fisher''s theory of dominance is discussed.