A partially purified fraction of extracted porcine pituitary glands which possesses lipolytic and adrenocorticotropic activity has been characterised. It consists of six adrenocorticotropin(ACTH)-like peptides (five of which have not been previously described) which were each purified by sequential reverse-phase (rp) HPLC. Their complete primary structures were determined following amino acid compositional analysis, extensive peptide mapping and partial sequencing. Four of the fragments represent the following ACTH fragments; ACTH(1-31), ACTH(7-34), ACTH(7-36) and ACTH(7-38). By combined analytical rpHPLC and an ACTH radioimmunoassay (with an antiserum exhibiting full cross-reaction with all six ACTH variants isolated here), evidence was obtained from analysis of extracts of whole pituitary that these fragments of ACTH exist in significant amounts relative to intact ACTH(1-39). This suggests that ACTH can undergo more extensive differential proteolytic processing than previously thought. These peptides were found to possess reduced or a complete absence of ACTH-like biological activity. Therefore the biological significance of this processing needs to be resolved. The other two fragments also resembled fragments of ACTH but each possessed the same, single amino acid substitution: a threonine replacing the arginine at the position corresponding to position 8 in the ACTH sequence and had the structures [Thr8]ACTH(1-31) and [Thr8]ACTH(7-31). They possess little ACTH-like biological activity. If these variants are derived from a variant ACTH, this would be a significant finding in view of the site of the amino acid substitution and the highly conserved nature of the ACTH primary structure. The possible physiological and genetic implications are briefly discussed. In this study attempts were also made to identify the DNA coding for the mutant ACTH sequence.