To determine the most sensitive index of iron deficiency in infants, a subset of samples was taken from a study of 267 Caucasian term infants recruited from Hamilton hospitals and randomized to standard term formulas containing 1.5, 3, 7 or 12 mg iron/L fed from birth to 12 mo with solid foods fed after 4 mo. Heel prick blood samples at 6 (n=76), 9 (n=78) and 12 (n=59) mo were analyzed for plasma Ft (RIA, Biorad, CAN), plasma TfR (ELISA, Ramco, TX), hemoglobin (Hb), hematocrit and mean corpuscular volume (MCV) (Coulter counter). There were no differences between iron groups for the latter 3 measures. Data (mean+SD) are summarized for Ft (analyzed as log transformations for nonnormal distribution), TfR and log TfR/Ft ratio (different letter superscripts indicate group differences, p < 0.05). Age, mo 6 9 12 6 9 12 6 9 12 Iron,mg/L Ft (μg/L) TfR (μg/mL) -log TfR/Ft 1.5 20±3a 20±2 20±2 9±211±2a10±2 0.4±0.4a 5.3 ± 0.3 0.4±0.3 3 40±2b 32±2 25±2 9 ± 2 9±1'10+2 0.7+0.3 0.5+0.20.4+0.2 7 40±2' 25 + 2 32il 9+2 9 + 2' 10+2 0.6+0.2' 0.4+0.3 0.5±0.2 12 63±2b 32±2 32±2 8±1 10±1b 10±1 0.9± 0.2a 0.5 ± 0.2 0.5 ± 0.3 Plasma Ft and log TfR/Ft similarly detected lower iron stores only at 6 mo in the 1.5 mg iron/L group. At 9 mo, plasma TfR indicated higher cellular iron needs in the 1.5 mg/L versus other groups. The overall incidence of Ft < 3rd %ile of normal values for age (Saarmen & Siimes, 1978) was 11%, with a greater (p<0.05) proportion m the 1.5 mg iron/L group at 6 and 9 mo. No case of iron deficiency anemia defined by Hb < 110 g/L plus Ft < 10 fig/L or MCV <70 fl was detected. In infants, use of TfR or log TfR/Ft ratio does not provide greater sensitivity than plasma Ft in detection of mild iron deficiency. Low iron formula when combined with solid foods likely provides adequate dietary iron to prevent anemia in infants through the first year.