SummaryAlloantibody was raised in PVG/c rats against DA rats. The catabolism of alloantibody in normal PVG/c rats and PVG/c rats tolerant to DA rats was followed after intravenous injection. A haemagglutination assay was used to detect alloantibody activity. The half‐life time of alloantibody IgG2a in the normal PVG/c rats was 10 days. On the other hand, in the PVG/c rats tolerant to DA rats, the half‐life of alloantibody lgG2a varied. The rats could be divided into three groups. Group A: alloantibody IgG2a was eliminated from the circulation very rapidly. Group B: alloantibody IgG2a disappeared from the circulation relatively rapidly and was maintained at a low level in the plasma but then increased to a high titre. This peculiar pattern of Group B seems to be due to the endogenous production of alloantibody. In this case abrogation of tolerance took place. Group C: the pattern of elimination of this group was almost identical to that of normal rats.The biological significance of the plasma decay of alloantibody IgG2a in the normal and homograft tolerant rats has been considered. It is concluded from the present results that alloantibody might be important in some cases for the maintenance of homograft tolerance.