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THE insulin-like growth factors were discovered on the basis of their ability to stimulate cartilage sulfation and to replace the “sulfation factor activity” of GH, as determined using an in vivo assay, in an in vitro test system (1). The biological significance of this finding was quickly expanded beyond the study of cartilage sulfation to include stimulation of DNA synthesis (2), proteoglycan synthesis (3), glycosaminoglycan synthesis (4), and protein synthesis (5). Most of these studies used tissue preparations such as isolated diaphragm, cartilage, or epididymal fat pads to study biologicalactivity. In recognition of its generalized pleiotypic actions, in the early 1970's sulfation factor was renamed somatomedin (mediator of the effects of somatotropin) and was included in the emerging classification of broad spectrum growth factors along with platelet derived growth factor, fibroblast growth factor, and epidermal growth factor (6). During the period in which the biological actions of sulfation factor were being characterized, parallel studies were initiated that attempted to define factors in serum that could stimulate insulin-like effects. Because these factors were known to be distinct from immunoreactive insulin, their bioassay was based on insulin-like actions that could not be abolished by the simultaneous addition of anti-insulin antibody, and the factors were termed nonsuppressible insulin-like activity (NSILA) (7). Initial biological studies of both NSILA and somatomedin were based on the use of crude serum extracts. Early attempts to extract these factors from tissues were generally not successful, and by 1970 it had been concluded that no concentrated organ source for them existed (8). However, highly purified preparations of both factors were prepared from serum extracts.