Denaturing solvents have been used to determine the molecular weight of the adrenocorticotropic hormone (ACTH) activity in mouse pituitary, in an ACTH secreting mouse pituitary tumor cell line (AtT-20/D-16v), and in the tissue culture medium from the pituitary tumor cells. ACTH activity was quantitated by radioimmunoassay and by bioassay. It is possible to utilize guanidine hydrochloride or sodium dodecyl sulfate in characterizing the multiple forms of ACTH because treatment of porcine ACTH (the 39 amino acid polypeptide form of ACTH, alpha(1-39)), pituitary extracts, tumor cell extracts, and tumor cell tissue culture medium with these denaturants does not diminish the immunological ACTH activity. Based on gel filtration in the presence of guanidine hydrocholoride, extracts of the pituitary tumor cells and the mouse pituitary contain three distinct molecular weight classes of ACTH activity. The major form of ACTH has a molecular weight similar to alpha(1-39) (molecular weight 4000-5500), but there are significant amounts of two higher molecular weight forms of ACTH: molecular weight 6500-9000 and molecular weight 20,000-30,000. The 6500-9000 molecular weight form of ACTH is the major form of ACTH in the tissue culture medium; there is no peak of alpha(1-39) size ACTH in the medium. In the radioimmunoasay all three forms of ACTH generate competitive binding curves parallel to that of porcine alpha(1-39); in the bioassay (stimulation of steroidogenesis in a mouse adrenal tumor cell line) the dose response curve for each of the molecular forms of ACTH is parallel to that for porcine alpha(1-39).