An electrophoretically HbA-like hemoglobin component is produced in increasing amounts during storage in hemolysate preparations from macerated tissue (liver, kidney, spleen) of fetuses. Within twenty four hours after hemolysate preparation the "fast moving" fraction increases up to 40 per cent of total hemoglobin, while the concentration of HbA remains constant (5 - 7%) in hemolysates obtained from peripheral blood of the same donor individuals. By structural studies (fingerprint and aminoacid analysis) the HbA-like component was identified as an artefact of HbF, characterized by the absence of the C-terminal arginine of the alpha chains. From experimental data it is concluded, that the break down product results from a digestion of HbF by carboxypeptidase B, the enzyme being released from the macerated tissues. Analogous to a modification of HbA, i.e. Hb Koelliker (alpha2 minus 141 Arg beta2), the structure of the degradation product of HbF is alpha2 minus 141 Arg gamma2 (HbF Koelliker).