The rare phenotype Gm(a–x+b + ) was studied in 3 Danish families. The Gm type determinations comprised the factors Gm(z), (a), (x), (f), (g), (b°), (b1), and (b3). In family No. 1 and 3 and in some members of family No. 2 also the Gm(n), (s), (t), (c) and (b4) types were established. The gene complex giving rise to the phenotype Gm(a–x+) probably is GmP Gmí, c. The origin of this gene complex is discussed, and its frequency in the Danish population is esti