Competitive tracer binding studies using radioiodinated insulin-like growth factor-I and -II (125I-labelled IGF-I and 125I-labelled IGF-II) together with size exclusion chromatography and IGF-I affinity chromatography have been used to characterize IGF binding protein activity in ovine tissue fluids. Binding proteins of greater than 200, 150 and 40-50 kDa were revealed in these studies and shown to be widely distributed in body fluids. Thus, the greater than 200 kDa binding protein, which is IGF-II specific, is present in plasma from mature sheep, colostrum and follicular fluid as well as fetal sheep plasma. This may be the ovine equivalent of the soluble type-2 IGF receptor recently identified in rat serum. The presence of a 150 kDa binding protein, of mixed specificity for IGF-I and IGF-II, in fetal and mature sheep plasma was confirmed in these studies. This protein, previously believed to be restricted to vascular fluids, was also identified in mammary lymph, follicular fluid and as a minor component in vitreous humor. Binding proteins of 40-50 kDa were revealed in every fluid tested and multiple variants with distinct specificities were also suggested. This was investigated by IGF-I affinity chromatography using mature sheep plasma. Following passage through the affinity adsorbent, binding of 125I-labelled IGF-I to proteins in the 40-50 kDa region was abolished but when 125I-labelled IGF-II was used as tracer, a binding protein of 40-50 kDa was still observed. Thus sheep plasma contains at least two 40-50 kDa binding proteins. The competitive tracer binding studies indicated that one such protein demonstrates mixed specificity for IGF-I and -II while the other strongly favours IGF-II.