Six women with a history of unexplained infertility or early miscarriage and who had failed to normalize their red cell magnesium (RBC-Mg) levels after four months of oral magnesium supplementation (600 mg/day) were investigated for red cell glutathione peroxidase activity (RBC-GSH-Px) and were compared with six age-matched women with a history of unexplained infertility or miscarriage who did normalize their RBC-Mg levels on magnesium supplementation. The six non-normalizers had significantly lower (P < 0.0001) RBC-GSH-Px levels than the six normalizers. After a further two months of 200 micrograms daily oral selenium as selenomethionine and oral magnesium supplements, all six women normalized their RBC-Mg (P < 0.0001) and RBC-GSH-Px (P < 0.0001) levels. All 12 previously infertile women have produced normal healthy babies all conceiving within eight months of normalizing their RBC-Mg levels. The possible relevance of these findings to the medical treatment of infertility is discussed.