The influence of pituitary adrenocorticotropic hormone (ACTH) and growth hormone (somatotropin, STH), singly and in combination, has been studied in normal, young adult rats, with respect to antibody formation against Fraction IA of Pasteurella pestis.

When ACTH was administered during the period of immunization, in a daily dose just sufficient to prevent body weight increase relative to the non-treated, immunized controls, serum antibody levels against the specific antigen employed were significantly depressed.

The administration of STH alone resulted in a marked increase in body weight. The increase in antibody level was not significant at the 5 per cent level when compared with the control values.

The same dosage of STH given simultaneously with ACTH maintained body weight at a level slightly above that of the controls, and resulted in an effective counteraction of the antibody depression produced by the latter hormone.

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