The solubility of sareoplasmic protins (Sp-P) of red meat fish, such as mackerels or dolphins, was examined at ionic strengths ranging from O.O to O.3 by diluting the muscle press-juice (PJ) to 1/22.5 with different concentrations of NaGl in the presence or absence of myofibrils (Mf). At I≥0.15 the proteins of the PJ all remained in solution, but when the ionic strength was lowered below 0.1, precipitations occurred whether Mf were present or not. The late of the precipitation, however, was greatly influenced by Mf; in the case of the Mf-free PJ, only 5-6% of the total Sp-P were insolubilized after the 22.5-fold dilution with deionized water, while in the case of the Mfcontaining PJ, 58% (incommon mackerel) and 43% (in horse mackerel) were precipitated at the same ionic strength conditions. The proteins precipitating together with Mf in the latter case were identified as the main component of the first (K_av=0.2-0.3) of the two myogen fractions separated by gel-filtration on a Sephadex G-200 colume. These results indicate that the low extractability with freshwater of Sp-P from the red-meat fish muscle, which is accepted as one of the distinguishing characteristics of this type of fish, is not due to a high globulin content in the Sp-P, but to the myogen-Mf interactions occurring at the low ionic strength condition.