Histones are the basic proteins complexed with DNA in the chromosomes of eukaryotic organisms. A previously undescribed histone (histone T.) was discovered in chromatin prepared from rainbow trout (Salmo gairdnerii) testes. Histone T was purified by selective extraction and ion-exchange chromatography on CM-cellulose. The homogeneity of this protein was examined by polyacrylamide disc gel electrophoresis, SDS disc gel electrophoresis, urea starch gel electrophoresis and gel filtration chromatography. Histone T was homogeneous as judged by the above different criteria. The molecular weight was found to be 14,500 by the mobility of formylated or acetylated histone Ton SDS gel electrophoresis. The amino acid composition and the N-terminal amino acid were determined. Peptide maps of the tryptic peptides of histone T and the major histones indicated that histone T is not a degradation product of the major histones. Details of the mechanism of initiation of protein synthesis in eukaryotes have only recently been discovered. Protamines are the small, highly basic proteins complexed with DNA in the mature sperm cells of most higher animals. The synthesis of protamine in trout testes was studied. Cell suspensions prepared from trout testes at the protamine stage of differentiation were incubated in vitro and were found to incorporate intact, isotopically labelled methionine into the N-terminal sequence, Met-Pro-Arg..., methionine residue is removed from protamine after chain completion. Enzymatic activity capable of cleaving the dipeptide, Met-Pro, was found in testis cell fractions. The amino group of methionine incorporated into ribosome-bound nascent protamine was not blocked. Methionine incorporation was extremely sensitive to inhibition by cyclo-heximide. The evidence obtained indicates a role for methionine in the initiation of protamine biosynthesis in the trout, a eukaryote / Medicine, Faculty of / Medical Genetics, Department of / Graduate