Serum somatomedin-like factor (Sm) and growth hormone (GH) were determined in normal and dwarf Leghorn chickens during the growth period from 7 to 30 weeks of age. The serum Sm activity was reduced with the advance in age, although there was no distinct decrease in serum GH concentrations by strain or sex. In both strains, serum from male showed significantly greater Sm activity than that from females. In contrast, the GH level in male serum was lower than that in females. In dwarf chickens, serum GH levels were comparable to those in normal chickens but Sm activity was extremely low, from 1% to 33% of the normal male serum. The serum level of inorganic sulfate, butanol extractable inhibitor, and free fatty acids in the dwarf bird was in the normal range, indicating that the low Sm activity was not attributable to a change in these factors. The response to Sm of cartilages from both dwarf and normal chickens was similar. The results indicate that the cause of the dwarfism is not poor-response of cartilage to Sm, but mainly low Sm activity during the period of growth, suggesting a possible Laron-type dwarfism in chicken.