The direct and indirect C1 fixation and transfer techniques were used to estimate the amounts of complement-fixing and non-complement-fixing IgM antibodies present in serial samples of serum taken from individual guinea pigs after immunization with rabbit erythrocytes, sheep erythrocytes, boiled sheep erythrocyte stroma, or bovine γ globulin. In most cases there was an increase in the relative proportion of non-complement-fixing IgM with time after immunization, and the ratio of non-complement-fixing antibody to complement-fixing antibody ranged from approximately 1 to greater than 10. Most antibody preparations contained several times as much non-complement-fixing IgM as complement-fixing IgM.