The ability of classical hormones or extract from adult chicken tissues to replace or influence the activity of chicken skeletal muscle cell trophic factor was investigated. Pituitary gland extract did not replace the trophic factor in a range equivalent to the serum concentrations of somatotropin, although high concentrations of such extract showed a significant ability to mimic trophic-factor activity. Insulin, triiodothyronine, testosterone, dihydrotestosterone (Stanolone), and estradiol did not show any ability to mimic trophic-factor activity. Both pituitary extract and insulin showed a potentiating effect on the trophic factor when added to the medium; however, the concentrations necessary for this effect were too high to be considered physiologic. Triiodothyronine, testosterone, Stanolone, and estradiol did not potentiate the trophic factor; indeed, at higher concentrations, these hormones actually suppressed the activity of the trophic factor. Every tissue extract examined showed, to a greater or lesser extent, the ability to potentiate the trophic factor.