A bioassay for measurement of human growth hormone based upon the observation that normal human serum, in vitro, will favor the incorporation of S-35 into immature rat cartilage is described. The factor in serum is called "sulfation factor." Sulfation factor determinations were performed on 27 normal children, 15 with hypopituitarism, 26 dwarfs without demonstrable endocrine or other diseases, 11 children with a variety of non-pituitary diseases causing dwarfism, and 1 adult with acromegaly. The mean activity (± S. E.) of sulfation factor was 0.84 ± 0.057 S. F. units in the normal children, 0.45 ± 0.024 S. F. units in idiopathic hypopituitary patients (p < 0.01), 0.95 ± 0.054 S. F. units in dwarfs without evidence of endocrine or other recognizable diseases, 0.82 ± 0.057 S. F. units in dwarfs with well recognized nonpituitary disease, and 2.2 S. F. units in one adult with acromegaly. There was an excellent correlation between sulfation factor and other laboratory determinations of pituitary function. Hypophysectomized rats receiving bovine growth hormone showed substantial increase of sulfation factor as compared to control hypophysectomized rats. Sulfation factor activity appears to be a valid and useful indicator of growth hormone activity in serum.