With use of a recently developed method for determining relative levels of IgG1 and IgG2 class antibodies of a given specificity within an unfractionated serum, it has been possible to examine anti-BALB/c antibodies in the early and late part of an immunization with allogeneic spleen cells. At about 6 days after primary immunization of CBA or C3H mice with BALB/c spleen cells, suppressive antibodies can be measured in the sera of the animals. About half of these are attributable to IgM class, and this contribution decreases to zero by the 12th day. The remaining suppressive antibodies are of IgG2 class and these increase in concentration until day 8 or 12, or begin to decline between day 8 and day 12. Anti-BALB/c antibodies of IgG1 class have not yet appeared on day 6, but thereafter appear and increase in concentration. Thus, antibodies of IgG1 class begin to appear after those of the IgG2 class and may still be increasing after the IgG2 class antibodies have begun to decrease. In the latter part of a secondary immunization of mice of these strains with BALB/c spleen cells, after the time when antibody of IgG2 class has stopped to increase in concentration, antibody of IgG1 class is continuing to increase and may even continue to increase after IgG2 class antibody has begun to decrease in concentration. Thus, the synthesis of IgG1 class antibody begins later and continues later than that of IgG2 class. The implications of this sequence for our data on various effects of anti-H-2 antibodies on retention of skin allografts are discussed.