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Phosphatidic acid (PA) from swine and beef RBCs was isolated by chromatography on silicic acid columns. It comprised about 1 per cent of the total lipid phosphate in RBCs, but was eluted nearly pure from columns. An uncharacterized inositide accounted for 5 to 10 per cent of the phosphate in the PA-containing fraction. When cells were incubated with HP(32)O(4) (=), the fraction containing PA became more radioactive than any of the other fractions obtained. However, analysis of the labeled material by paper chromatography showed that most of the P(32) was in the inositide, not in PA. With the assumption of kinetic homogeneity for cellular PA, compartmental analysis of the kinetics of tracer incorporation showed that PA turnover is 3 to 4 orders of magnitude too slow to account for sodium extrusion by these cells.