Low molecular weight peptides are linked to the chromatin DNA of several tissues, from which they can be dissociated by alkaline extraction. They show very high specific activity in the control of transcription "in vitro". In this work the biochemical properties of controlling transcription peptide effectors isolated from trout testis DNA are reported. The purified peptides prevailingly contain glutamic acid, serine, aspartic acid, glycine and alanine. Studies of the peptide structure by N-terminal analysis using the dansyl chloride procedure was unsuccessful, suggesting the presence of a blocked NH2 group. At the same time the active peptides cannot be digested by carboxypeptidases. The gel filtration of the chromatin peptidic fractions on Sephadex G-25, Trisacryl GF05 or Sephadex G-15 shows that the active peptides elute as a single major peak with an elution volume corresponding to a molecular weight of about 1000. The paper electrophoresis performed at different pH and ionic strength shows that the chromatin peptides are separated in two fractions. One of them is strongly acidic and migrates towards the positive pole until pH 1.9, indicating the presence of phosphoric residues which probably exert an important role in the control of transcription "in vitro". The chromatin peptides are further purified by Sephadex G-10 and high performance liquid chromatography. The amino acid analysis of the purified peptides are reported.