Bovine GH and two fragments which were obtained by dissociation of a limited tryptic digest of the hormone stimulated protein, RNA, and DNA synthesis of contact-inhibited human fibroblasts. The stimulation of protein, RNA, and DNA synthesis was similar for the test substances. Maximal stimulation was noted at 10 nM. At that concentration, protein, RNA, and DNA synthesis were respectively increased 1.80, 1.42, and 1.37 times by GH; 1.93, 1.27, and 1.46 times by A-I (the larger fragment); and 1.99, 1.26, and 1.33 times by A-II (the smaller fragment). The action of GH, A-I, and A-II was similar to that of fetal calf serum, but was distinguished by the time course of stimulation and by the magnitude of the response. For example, GH, A-I, and A-II produced their earliest detectable effect at 10 h for protein synthesis, 22 h for RNA synthesis, and 26 h for DNA synthesis. On the other hand, serum stimulated protein, RNA, and DNA synthesis at 6, 10, and 16 h, respectively. These data show that human fibroblasts respond equally to GH, A-I, and A-II and suggest that there may be more than one "active site" in the GH molecule. Lastly, human fibroblasts may represent a useful system to study the actions of GH in vitro.