2 new cases of triosephosphate isomerase (TPI) deficiency associated with severe haemolytic anaemia in 2 unrelated Italian families are described. Only 1 case was extensively investigated. TPI deficiency was detectable in erythrocytes, leucocytes, platelets and plasma. The mutant enzyme showed normal Km for GAP and increased Km for DHAP, with an higher than normal equilibrium constant, decreased thermostability, and abnormal electrophoretic pattern due to the lack of the fastest moving component. The immunological characterization revealed a lower than normal inactivation by specific antiserum, while the double immunodiffusion pattern and the precipitin curve were normal. Lymphocyte, granulocyte and platelet functions were impaired.