Muscle expression of G6PD deficiency has been investigated in Mediterranean, Seattle-like and A-variants. G6PD activity was detected in samples obtained from biopsies on the quadriceps muscle of seven males and one female. The type of genetic variant was determined by molecular analysis of DNA, extracted from blood samples. All variants showed the enzyme defect in muscle. A statistically significant relationship was found in the activity of G6PD between erythrocytes and muscle of the male subjects (r = 0.968; p = 0.00008). The equation for the best fit line was: Y = 0.390X + 0.198. The results suggest that, for a given variant, the extent of the enzyme defect in muscle may be determined, using this equation, from the G6PD activity of erythrocytes.