Methods for selective reduction of the disulfide bonds in ovine prolactin have been developed. Cleavage of all three disulfide bonds abolishes biological activity and denatures the hormone. Reduction-carbamidomethylation of one or two of the disulfide bridges does not diminish the biological activities in the pigeon crop-sac and mouse mammary gland bioassays. When compared to the native hormone, monomers of these two partially reduced -carbamidomethylated derivatives also show only modest changes in properties measured by exclusion chromatography, circular dichroism, and immunological cross-reactivities. However, cleavage of cystines 4-11 and 191-199, followed by carbamidomethylation destroys the biological activity of this derivative in a teleost fish bioassay (Gillichthys urinary bladder). The modification of histidine residues of ovine pituitary lutropin by rose bengal sensitized photooxidation has been investigated. The destruction of an average of one histidine out of six lead to 90% of biological activity as examined by the in vitro steroidogenic response in the rat Leydig cell assay. Further modification of an average 2-3 histidine residues reduced the biological activity to less than 1% of the native lutropin. The modified lutropin was incapable of inhibiting the native lutropin induced steroidogenesis. Gel filtration experiments and polyacrylamide disc gel electrophoresis patterns indicated that no dissociation of the molecule into subunits occurred.