The somatomedins comprise a family of insulin-like peptide growth factors, whose concentrations in postnatal life are growth hormone-dependent, that stimulate sulfate uptake by cartilage and that stimulate the proliferation of a variety of types of cultured cells. Several lines of evidence now support the concept that the somatomedins stimulate fetal growth: 1) they act as mitogens for a number of embryonic and fetal derived cells grown in culture; 2) specific plasma membrane receptors for somatomedins exist in many fetal tissues; 3) multiple fetal tissues appear to be capable of somatomedin synthesis; 4) in some studies blood somatomedin concentrations correlate with birth size. It is proposed that the somatomedins might act at or near their sites of synthesis, and that under the influence of placental lactogen, as well as other factors, they have a significant stimulatory effect on both organ-specific and generalized somatic fetal growth. A hypothesis that integrates the possible growth-promoting actions of both classic hormones and growth factors in the fetus is presented.