SUMMARYUsing specific immunoprecipitation of M‐type phosphofructokinase and assay of immuno‐precipitate enzyme activity, it was possible to detect some M‐type enzyme in normal blood cells and fibroblasts, although this isozyme represents a very small part of total phosphofructokinase. White blood cells and cultured fibroblasts from a patient with hereditary muscle phosphofructokinase deficiency showed normal phosphofructokinase activity and electrophoretic pattern; direct immunoneutralization results were also normal. Nevertheless, it was possible to prove the defect in these cells using the immunoprecipitation method: no active immunoprecipitates could be obtained with anti M‐type antibody.Thepatient's red blood cells had a reduced phosphofructokinase activity which was only neutralized by anti L‐type antiserum. Total purification of partially deficient red cell phosphofructokinase confirmed that this enzyme only consisted of L‐type subunits while, under normal conditions, both L‐ and M‐type subunits are observed.The possibility of detecting specific enzyme defects in apparently non‐affected cells could be of practical importance, especially in p