Aluminum (Al) retention in the body can cause metabolic bone disease. This disorder is characterized by reductions in the number of osteoblasts, a feature that suggests a disturbance in bone cell proliferation or differentiation. Because Al as well as iron (Fe) can bind to transferrin (TF) in plasma, the role of TF as a modifier of osteoblast proliferation was examined in UMR-106-01 osteoblast-like cells by measuring the incorporation of tritiated thymidine ([3H]-TdR) into DNA (counts.min-1.microgram cell protein-1, means +/- SE) during 48-h incubations in serum-free medium (SFM). In the absence of TF, DNA synthesis decreased when media levels of Al exceeded 6-10 microM. The mitogenic response to physiological levels of unsaturated TF (apo-TF) was attenuated however during incubations with TF that was partially saturated with Al (Al-TF). A similar inhibitory response was seen in cells incubated with the antiproliferative agent gallium (Ga) when added to SFM as partially saturated Ga-TF. TF produced a shift to the left in the inhibitory dose-response curve to Al in osteoblast-like cells; thus, DNA synthesis decreased at substantially lower media concentrations of Al in cells grown in SFM containing partially saturated Al-TF. The results indicate that TF is an important determinant of the inhibitory effect of Al on DNA synthesis by osteoblast-like cells at the micromolar levels of Al that can occur in plasma in vivo.