The immunosuppressive properties of rabbit and burro anti-mouse lymphocyte sera (ALS) before and after absorption with sheep erythrocytes (SRBC) have been studied. Five out of nine absorbed sera retained their ability to depress splenic plaque-forming cells (PFC) and serum hemagglutinin and hemolysin titers of mice immunized with SRBC. Four sera completely lost their immunosuppressive activity. Seven of the nine ALS had low titer anti-SRBC antibodies, but their complete removal by absorption did not reduce the potency of five of these sera indicating that their immunosuppressive activity was independent of passively transferred heterophile antibodies. The ability of absorbed ALS to prolong mean survival time of A/J to C57BL/6 skin allografts was significantly reduced when compared to that recorded in mice treated with unabsorbed ALS (16.2 vs 23.8 days). These findings are most consistent with the interpretation that immunosuppressive antilymphocytic antibodies had been removed from ALS by absorption with SRBC.

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