Erythrocytes of both G-6-PD deficient humans and Dorset sheep, an animal model with an erythrocyte G-6-PD deficiency, both responded in a dose dependent manner to the oxidant stress of MOHP as measured by decreases in G-6-PD activity, increases in METHB levels and decreases in GSH. However, the human G-6-PD deficient erythrocytes were considerably more sensitive to the formation of METHB than the sheep erythrocytes while the reverse was the case for the GSH parameter. The results suggest a qualitative difference in the response of sheep erythrocytes and human G-6-PD deficient erythrocytes to MOHP that seriously questions the value of the sheep erythrocyte as a quantitatively accurate predictive model.