The electrophoretic variant of human LDH, Calcutta-1, occurs at phenotypic frequencies of 0-4% throughout India. The variant was examined by various electrophoretic techniques and by heat stability studies. The LD1 (B4) isoenzyme was purified from normal and variant bloods by affinity chromatography and ion-exchange chromatography. A minimum of five Calcutta-1 LD1 bands was demonstrated by isoelectric focusing. Electrophoresis of variant LD1 in high-molar urea-acrylamide denaturing gels resulted in two Calcutta-1 B subunit bands, while normal gels yielded only a single band. Homozygote Calcutta-1 LDH from red cells demonstrated a decreased heat stability, while heterozygote variant LDH showed a normal heat stability. This effect was confirmed when purified LD1's were compared. Evidence is presented suggesting a B-subunit variant showing thermolability in the homozygous form.