Serum ferritin levels were determined in 170 healthy Norwegian women (18-48 y, median age 36 y) including 23 blood donors. Exhausted iron stores, defined by serum ferritin levels less than 17 micrograms/l, were found in 21.8% of the non-donors, and in 30.4% of the donors. Women with serum ferritin levels less than or equal to 20 micrograms/l participated in a bioavailability study. They were randomized to one of three groups and given one of three different low dose iron supplements (18-20 mg iron per day) for six months. One of the supplements contained heme iron and non-heme iron, the other two contained non-heme iron only. Mean serum ferritin increase was significant for two of the supplements, the one containing heme iron giving the best result. All the supplements resulted in a significant decrease in TIBC.