This paper describes the isolation, purification and full characterization of the protamine from Sardinops sagax. The protein was purified by both gel filtration and ion exchange chromatography and its amino acid composition, electrophoretic behaviour and sequence were determined. We postulate that the protamine exhibits microheterogeneity with a major and a minor component. The molecular weight for the major component is 4377, as calculated from the sequence. The N-terminal amino acid is proline and the C-terminal is arginine. The amino acid composition is: Thr-Ser-Glx-(Pro)2-Gly-(Ala)3-(Val)2-(Lys)2-(Arg)20 The secondary structure for the protamines was also predicted and a revision of our previously published results in this area is discussed. We have called this protamine Sagaxine and an overall comparison with the hitherto described fish protamines is also made.