Progesteron and estradiol, administered in doses equivalent to those used for therapy caused a marked transient reduction of the thymus mass, but did not affect the cellularity of other lymphoid organs. Humoral and cellular immune response of the hormone-treated mice was normal at the time of thymus involution. The same was true for the stem-cell differentiation capacity. The remaining thymus cells after hormone treatment showed increased DNA-synthesis.

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A substance immunologically cross-reactive with insulin (SICRI) was isolated and purified from murine melanoma B16. Biochemical and biological data provide evidence that SICRI and insulin are two distinct biologically active agents. In vitro, SICRI has stimulated the DNA and protein synthesis, cell growth or colony-forming ability of 19 normal and transformed cell lines of human and rodent origin. It indicates that SICRI is a potent nonspecific growth factor. The most pronounced stimulatory effect of SICRI on proliferative capacity of cells is observed with cells at G_o/G₁ point of the cell cycle.