An Escherichia coli JM109 clone containing a plasmid, pOGHe101, based on pUC8 and the ovine GH (oGH) cDNA sequence, showed very high expression (up to 25% of total cell protein) of an oGH analogue (oGH1) after induction. oGH1 was found in the particulate fraction of induced bacteria, where electron-dense granules could be seen by electron microscopy. A simple method for the purification of oGH1 is described. The particulate fraction isolated from sonicated bacteria was dissolved in 6 M guanidinium chloride containing dithiothreitol. After threefold dilution the proteins were reoxidized by gentle stirring overnight in air. Soluble renatured protein, recovered after dialysis, was further purified by ion-exchange and gelfiltration chromatography. Purified oGH1 had an M r of 22 000, an isoelectric point of about 6·7 and an N-terminal sequence corresponding to that of oGH, with an extension of eight amino acids replacing the N-terminal alanine. oGH1 behaved similarly to authentic bovine GH in a radioimmunoassay, a radioreceptor assay and a weight-gain assay in hypophysectomized rats. Thus the renatured hormone appears to be correctly folded and the N-terminal extension has little or no effect on biological activity.