More than 85% of the complete amino-acid sequence of the alpha-class glutathione S-transferase omega (GST omega) of human liver, described for the first time in this communication, show that GST omega is a heterodimer of two closely related novel alpha-class GST subunits. The sequences of these subunits, omega 1 and omega 2, have over 97% homology between them and are also highly homologous to the two alpha-class subunits characterized previously. Characterization of these two novel alpha-class subunits described in this report would explain the molecular basis for high degree of heterogeneity observed among the alpha-class human GSTs.