The `activity' of rabbit anti-mouse ALS was measured in terms of its power to prolong the life of A-strain homografts on CBA mice. Antisera raised by lymphocytes from mice of other strains were active in CBA mice, and conversely antisera raised by CBA lymphocytes were active in mice of other strains. All active sera were cytotoxic to mouse lymphocytes in the presence of mouse complement, but not all cytotoxic sera were active. ALS raised in species (chicken, duck) of which the sera do not bind with mouse complement were inactive in mice. Neither cytotoxicity nor the power to circumvent graft-versus-host reactions give adequate measures of the activity of impure antisera. Good antiserum (`two-pulse' ALS) for use in mice could be raised in New Zealand White rabbits by the intravenous injection of 10(9) mouse thymocytes on two occasions 14 days apart, followed by bleeding out 7 days after the second injection. Further injections of antigen increased the titre of undesirable contaminants in ALS (e.g. red cell agglutinins) but usually did not increase (and sometimes lowered) activity. ALS raised with adjuvants, though powerful, was highly toxic and required extensive absorption
