The conformations of porcine somatotropin and human choriomammotropin have been studied using circular dichroism (CD) and the results compared with spectra of human, murine, ovine, and bovine somatotropin. The far ultraviolet CD spectra of the six proteins were similar, and each spectrum was analyzed using constrained linear least squares. The following average percentages of alpha-helicity, beta-structure, and aperiodic (nonhelical) conformation were obtained: 57, 6, and 37, respectively, based on a standard protein reference set, and 42, 22, and 36, respectively, based on poly-L-lysine as reference. Thus, the estimated secondary structure is strongly dependent upon the reference data used. Interestingly for these similar proteins, it appears that over 60% of the residues are part of ordered secondary structure and less that 40% are in an aperiodic conformation. The near ultraviolet CD spectra of these hormones were similar in many respects, although certain significant differences were observed, particularly in the sign of various extrema. These spectral differences probably reflect non-identical microenvironments of the aromatics and disulfides, arising from differences both in amino acid sequence and local conformation.