H-glycylhistidyllysine-OH (GHL) is a tripeptide found at approximately 200 ng/ml in human plasma in association with the albumin and o-globulin fractions [ 1,2 J. When added to culture medium at 1 S-100 ng/ml, synthetic GHL promotes the proliferation of hepatoma cells ]2,3], lymphocytes [4] and T-strain mycoplasma [5]; maintains the viability of normal hepatocytes [2,3], eosinophils [6], and macrophages [7]; inhibits the growth of glial cells [8]; and supports the growth and differentiation of neurons ]8] and ascaris larvae [9]. At higher concentration, the tripeptide inhibits L929 cell growth (500 ng/ml) [lo] and maintains the viability of mast cells (20 pg/ml) during degranulation tests [ 1 I]. In general, GHL acts to reduce or eliminate the amount of serum required for culture of the various cell types or organisms [2-4,6-9,111. Although the mechanism of action of GHL is unknown, the tripeptide is co-isolated with copper and iron [ 12,131, and acts synergistically with these transition metals to stimulate the growth and metabolism of hepatoma cells maintained in grows-l~it~g amounts of serum [ 12,131. In this paper we describe the effects of 9 synthetic analogs of GHL on DNA synthesis in hepatoma cell culture. The results indicate that 2 structural features are involved in the bioactivities of GHL: (1) the histidyllysyl linkage and (2) a glycyl residue in either terminal position.