Adjuvant arthritis was induced in rats by a single intracutaneous injection of Freund's complete adjuvant; after 9 to 12 days, all control rats developed polyarthritis. Administration of antilymphocyte globulin about the time of adjuvant injection produced marked inhibition of clinical disease. Some suppression was apparent even when ALG was started after the onset of arthritis. Rats receiving ALG remained conspicuously healthy compared to controls. Urinary findings and renal histology showed no evidence of nephritis. The results of serum corticosterone determinations made it unlikely that adrenal stimulation contributed to the actions of ALG.

Antilymphocyte preparations lowered peripheral lymphocyte counts and suppressed primary antibody responses to sheep erythrocytes, but had little effect on the skin reaction to PPD, even in rats protected from arthritis.

All rats given ALG developed antibodies to rabbit globulin; there was no evidence that ALG inhibited the appearance of antibodies to itself, and prior hyperimmunization of rats with rabbit globulin did not interfere with the biological activity of ALG subsequently injected.

Antisera produced separately against lymph node and thymus cells had identical properties with regard to agglutination of lymphocytes and thymus cells. Administered to rats, these preparations were equally potent in lowering lymphocyte counts and suppressing both adjuvant arthritis and the primary antibody response to sheep erythrocytes.

It is concluded that ALG, as used in these experiments, is a potent immunosuppressive agent without obvious toxic effects.