Thymic factor in human sera demonstrable by a cyclic AMP assay

NORMAL human serum has been reported to contain thymus hormone activity as detectable by a rosette inhibition assay 1 . It has been suggested that this circulating hormone may act through stimulation of cyclic AMP synthesis 2 . In addition, other thymic hormones isolated from the thymus, such as thymosin 3 and THF 4 , have been found to activate membrane adenylate cyclase and to increase cellular levels of cyclic AMP in thymocytes 5,6 . Thymopoietin, a thymic hormone the amino acid sequence of which has been established 7 , has also been claimed to induce T-cell differentiation by way of cyclic AMP 8 . Here we report on a human serum factor (SF) capable of increasing the level of cellular cyclic AMP in thymocytes; we also present evidence for the possible thymic nature of this factor.