Phosphofructokinases from rabbit muscle and rabbit liver were hybridized by dissociation at low pH followed by recombination at neutrality. Three hybrid species could be generated to give a total of five phosphofructokinase isoenzymes as detected by cellulose acetate electrophoresis. An identical five-band phosphofructokinase activity pattern was observed upon electrophoresis of rabbit adipose tissue extracts. It is suggested that the phosphofructokinases of liver and muscle differ in the structure of their monomeric units and that each enzyme is a tetramer composed of identical monomers. In adipose tissue, and apparently some other tissues as well, both types of monomeric unit are present. It is proposed that liver phosphofructokinase be identified as phosphofructokinase 1 and the muscle enzyme as phosphofructokinase 5.