Aluminum-induced osteomalacia is a frequent complication observed in patients on maintenance hemodialysis. However, it is not known whether there are direct effects of aluminum on osteoblasts, or alternatively, whether the observed changes are due to changes in PTH or other factors. We sought to determine the effect of micromolar levels of aluminum on osteoblasts using a well-defined cell line derived from a 32P induced osteosarcoma of rat. UMR 106-01, which is alkaline-phosphatase positive, responds to PTH, and synthesizes type I collagen. Aluminum exposure was controlled using tissue culture media with [Al+++] less than 1 μg/liter (40 nM), produced by precipitation of aluminum salts at pH 8.5. The effect of defined [Al+++], from 20 to 800 μg/liter (0.7 to 30 μM), was then determined by adding back aluminum while measuring DNA and protein synthesis. We found that aluminum depressed DNA synthesis, as determined by 3H-thymidine incorporation, by 60%, with half maximal effect at 20 μg/liter (740 nM) in cells at a density of 20,000/cm2. Alternatively, protein synthesis, as determined by 3H-leucine incorporation, did not decline, and in some cases increased. However, qualitative analysis of matrix proteins produced with and without 800 μg/liter (30 mM) [Al+++] showed no differences. Direct measurements of cell number and protein synthesis confirmed these findings. Al+++ does not alter the PTH-induced cAMP response of these cells. Thus, aluminum has a direct effect on cell division, and probably on protein synthesis, in this osteoblast-like cell line. These effects occur at levels of aluminum below those commonly contaminating tissue culture media, and thus are seen reproducibly only in media of defined [Al+++]. The changes seen are most consistent with a shift of cells, at [Al+++] over 1 μM, from cell division toward other cellular functions.