The background of the development of the work in isolating interstitial cell-stimulating hormone (ICSH) and follicle stimulating hormone (FSH) involved their isolation and purification. Ovine ICSH is a reversible dimer with monomeric molecular weight of 16,000 and consists of 2 subunits alpha and beta, each of which are individually inactive. The immunologic and biologic activity reside in the beta-subunit. The amino acid seuqences of ICSH-alpha and ICSH-beta are presented. The work on FSH is still in progress.