We studied new immunological and physicochemical properties of K-activin, immunotropic preparation from porcine skin isolated by the acetone method. The preparation restored the sensitivity of background rosette-forming cells in the spleen of thymectomized mice to the inhibitory effect of azathioprine in vivo and practically normalized serum thymic activity reduced in thymectomized mice. The molecular weight of proteins present in K-activin and previously detected by SDS-PAAG electrophoresis was determined by MALDI mass spectrometry