Growth-stimulating activity of plasma was measured, both as thymidine activity using a lymphocyte bioassay and as sulfation activity using a cartilage bioassay, in 72 constitutionally short children aged 1–6 years and 38 age-matched controls. Plasma transferrin was also measured. No significant differences were found between the two groups. However, short children aged 1–3 years did not have a higher thymidine activity than children aged 4–10 years, whereas in normals the thymidine activity was significantly higher in the first years of life. Thymidine activity was lower in female than in male constitutionally short chi