The levels of IgM were measured in sera from mice with different chromosome complements, including 39,XO mice and phenotypically male mice bearing the sex-reversed gene (Sxr) (XX,Sxr/+ and XY,Sxr/+). The mean values of IgM were found to be higher in normal female mice than in the males belonging to two different strains. This is in agreement with the higher mean serum levels of IgM observed in two groups of sera from normal human females and males. However, while we could confirm that the mean level of IgM was lower in 45,XO women than in normal females and comparable to the mean value of normal males, the same effect was not seen in 39,XO mice. In fact, the mean concentration of serum IgM in 39,XO mice was similar to that in normal females. Furthermore, it was observed that the mean values of serum IgM in the two groups of sex-reversed male mice (XX,Sxr/+ and XY,Sxr/+) were also not statistically different from those in normal males. Thus the role that the number of X chromosomes plays in the control of the serum levels of IgM is different in man and the mouse, in agreement with the observed different phenotypic manifestations, and in particular with the hormonal situations existing in X chromosome abnormalities in these two species.