The presence of three different growth hormones (GH) in molecular weight, “big big GH”, “big GH” and “little GH”, is reported here. Human pituitary extracts from an acromegalic patient and from a postmortum patient were fractionated into three fractions using gel filtration of Sephadex G-200. The major component of the pituitary GH was observed at the most retarded fraction (“little GH”), which was identical with the highly purified HGH in the elution pattern. The “big GH” was less retarded in the gel filtration and was eluted between the “little GH” and 4S fraction. The small peak of GH with the largest molecular weight (“big big GH”) was eluted between 19S and 7S fractions. The activity of these three GHs in different molecular weights was observed using GH radioimmunoassay. In refiltration of each fraction of normal human pituitary extract, these three components were isolated independently at each filtration. The conversion from “big big GH” to “big GH” or to “little GH” was observed during st orage at 4°C for 7 days and the similar conversion, from “big GH” to “little GH” was also observed in refiltration of “big GH”, however the reverse reaction from “little GH” to “big GH” was not observed. The physiological significance of these hormones of different molecular weights is not clear, but it is suggested that the immunoreactive OH with large molecular weight may be a form of the precursor or polymer.