Sodium chloride prevents drip in lightly brined fish muscle by causing the proteins to swell and to bind liquid firmly, and not by a surface sealing action. In whole and in comminuted muscle at its natural pH, free drip is almost completely inhibited, and expressible drip greatly reduced, by incorporation of about 1 per cent NaCl. On either side of an approximate "isoelectric zone" (about pH 4.5 to 6.0) addition of HCl or NaOH causes fish muscle to swell markedly and to withhold liquid against hydraulic pressure, peptization of the proteins occurring above and below about pH 10 and 2 respectively. Between about pH 4.5 and 7.0, from 1 to 3 per cent NaCl enhances swelling and liquid binding power of fish muscle, while below pH 4.5 it has an entirely opposite effect. Addition of NaCl effects an increase in pH of fish muscle below, and a decrease above about pH 4.5. In 1 per cent NaCl fish muscle does not swell appreciably; in 3 to 7 per cent swelling is at a maximum, while higher concentrations cause an apparent decrease in swelling which may be due in part to the peptizing action of the salt. Theoretical and applied aspects of the process of brining fillets are discussed.