The use of the EEL 140 atomic absorption spectrophotometer for the determination of magnesium in plasma, skeletal muscle, and bone is described together with an examination of the pattern of chemical interference in each type of sample. The sampling procedure and preparation of each kind of sample for analysis is detailed, and recovery studies on plasma bone and muscle are presented. The mean value for plasma magnesium in 42 normal men and women was 2.16 mg/100 ml ± SD of 0.08. For 30 male Wistar strain rats, the plasma value was 2.10 mg/100 ml ± SD of 0.18. In 15 normal postmortem human muscle samples, the mean magnesium content was found to be 93 mg/100 g of dry muscle, the series range being between 83 and 101 mg/100 g. The comparable value in 49 male rats was 125 mg/100 g dry weight ± SD 1.8. Bone magnesium in femur diaphysis of 20 male rats (average weight 200 g) was 563 mg/100 g dry weight ± SD 12.7. It is concluded that this instrument, when operated in the manner suggested in our study, is technically easy to use, and gives precise, sensitive, and reproducible results for magnesium in a variety of biologic material.