Many peptides contribute to the heterogeneity of immunoreactive adrenocorticotropin (ACTH) in man. The use of a radioimmunoassay (RIA) specifically directed against the C-terminal end of ACTH allowed us to study precisely the following four peptides: ACTH itself, corticotropin-like intermediary lobe peptide (CLIP) or ACTH(18–39) and their phosphorylated forms on Ser³¹. We have set up a highperformance liquid chromatography system that separates these four molecules in a single run, to establish their relative distributions in tumors responsible for Cushing's disease or for the ectopic ACTH syndrome, and to evaluate the possible interference of phospho-S^er on various RIA or immunoradiometric assay (IRMA) recognition systems for ACTH. In this system, alkaline phosphatase treatment shifted the retention time of the phosphorylated peptides to that of their non-phosphorylated counterparts. In three tumors responsible for the ectopic ACTH syndrome, CLIP peptides were predominant in two and phosphorylated molecules represented between 22% and 50% of immunoreactive materials. In five pituitary tumors responsible for Cushing's disease, ACTH peptides were predominant and the phosphorylated molecules varied between 35% and 75% in four of them. In the same tumor the ratios of phosphorylated to non-phosphorylated CLIP or ACTH were identical. The presence of phospho-Ser³¹ did not affect the recognition ability of two mid-ACTH and two C-terminal ACTH RIAs, nor of the ACTH IRMA (Allegro, Nichols).