Alloantibody-containing globulins of the mouse (CBA or C3H versus BALB/c), selected for relatively high titer of anti-BALB/c suppressive antibody, have been examined for their effect on BALB/c skin grafts in CBA or C3H mice rendered immunologically tolerant of BALB/c grafts by perinatal injections of (CBA × BALB/c) F1h spleen cells. Such globulin pools, obtained from ascitic fluid collected 11-13 days after secondary injection of BALB/c spleen cells and injected at the time of grafting, were found to cause complete rejection of the grafts between 10 and 20 days after grafting, the rejection by individual pools occurring largely within a relatively narrow range of 3-6 days. Globulins obtained from the same groups of mice later than 11-13 days caused rejection a few days later. It was also possible to cause rejection of 15 of 17 established BALB/c skin grafts after 2 months in situ on immunologically tolerant hosts, more than one-half of the 15 grafts rejected being distinctly smaller within 10 days after the beginning of the injections, and completely rejected within 16 days. Mice whose tolerated grafts had been rejected following the injections of the alloantibodies were then given second allografts of BALB/c skin to test for residual tolerance. All of these were retained for periods between 21 and 45 days.