Monomeric human growth hormone (hGH) and big hGH obtained by a new purification procedure from human pituitary glands were examined for a possible conversion into different molecular weight forms under in vitro and in vivo conditions. Sephadex G-150 gel filtration was used for separation. Big or monomeric hGH was incubated in hormone-free serum or injected into normal individuals in order to induce a peak of immunoreactive hGH. Specimens from early and late stages of the surge were subjected to gel filtration. It could be shown that not only the big hGH is converted to the smaller form but that monomeric hGH is also recovered in two higher molecular weight regions by gel filtration in vitro and in vivo experiments. A significantly higher conversion of monomeric to big hGH was found in vitro. This suggests an inhibition of big hGH formation from monomeric hGH in normal individuals. There was no apparent time relation of big hGH formation from monomeric hGH in vivo. Regarding the conversion of big hGH, more of this material remained in its original form in vitro. The immunoreactivity recovered in the region of the very large form was found to be higher in vivo. The formation of the monomer from big hGH was similar under in vitro and in vivo conditions.