A preparation of phosphofructokinase from rabbit skeletal muscle is described which exploits the association-dissociation properties of the enzyme. Phosphofructokinase to prepared is partially phosphorylated and may be fractioned into three distinct species with sedimentation coefficients of 30 S, 18 S and 13 S by chromatography of agarose gels, hydroxyapatite or DEAE-cellulose. Measurements of alkali-labile phosphate content (phosphoserine and/or phosphothreonine) show that fractions consisting almost exclusively of 30-S species and fractions consisting predominantly of 18-S and 13-S species contain approximately 0.15 and 0.29 mol of phosphate per phosphofructokinase monomer (Mr = 80000) respectively. The results are interpreted in terms of at least two 13-S components which differ in their phosphate contents and also in their self-association properties. The possible significance of phosphorylation is discussed.