The conformations of human, equine, and porcine pituitary prolactins, as evidenced by various optical properties, have been compared. The alpha-helix contents of all three proteins are essentially identical to each other (60 +/- 5%), as well as to prolactins isolated from other mammalian species. Direct absorption (zero and second-order), difference absorption, fluorescence emission, and circular dichroism spectra suggest that the majority of tyrosine and tryptophan side chains in these three proteins exist in very similar microenvironments within the folded forms of the hormones. Thus, the general conformational properties of these molecules are closely related to each other, and to other mammalian prolactins. Molar extinction and absorptivity values have been obtained at the absorption maximum of each species. In addition, a second molar extinction value has been determined at a particular wavelength found to be different for each, and which appears to be independent of the conformational state of the molecule. These absorptivities are useful in providing accurate prolactin concentrations in the 10(0) to 10(-2) mg/ml range. On incubation with the proteolytic enzyme thermolysin, all three hormones display an initial, short lag period during which little conformational change can be detected by difference absorption spectroscopy. For human and porcine prolactins, subsequent rates of proteolytically induced conformational collapse were found to be essentially identical. However, under similar conditions, equine prolactin loses its conformation significantly more slowly.