1. Chemical composition of erythrocyte HbA₂ was analyzed in order to clarify the mechanism of hemoglobin A₂ phenomenon. The paper reports the hemoglobin composition of erythrocyte HbA₂.2. The results of two-dimensional electrophoresis showed that erythrocyte HbA₂ contains two Hb components: one corresponds to HbA and the other most probably is hemolysate HbA₂.3. The results of one-dimensional re-electrophoresis showed definitly that erythrocyte HbA₂ is composed of hemolysate HbA₂ and HbA.4. Tentative conclusion: HbA₂ may be combined with HbA in erythrocyte, i.e. there is probably interaction between the two hemoglobins. This is perhaps the possible cause of hemoglobin A₂ phenomenon.