Mouse IgE was titrated in rats. The sensitization period was 2h. The results were consistent and corresponded to the titers obtained in young adult SJL mice using a sensitization period of 48 h. With longer sensitization periods in rats, higher antibody titers were obtained. The optimum sensitization period in rats was found to be 48 h. Old CFW mice are inadequate for mouse IgE titration. IgG₁ will not give a PCA reaction in rats. IgG₁ titers are higher in SJL or A/J mice than in young CFW mice, and markedly higher than in old CFW mice.