Six adult subjects were administered a series of manganese (Mn) tolerance tests to investigate the influence of various minerals on Mn plasma uptake. Oral loads given to all six subjects included 40 mg manganese alone, or with 800 mg calcium (Ca) as either calcium carbonate (CaCO3) or 545 ml 2% milk. Four of the subjects also received loads of 800 mg phosphorus (P), 2 mg copper (Cu), and 50 mg zinc (Zn) with the 40 mg Mn. Baseline Mn tolerance tests for all subjects produced a rapid increase in plasma Mn, followed by return to baseline. The addition of Ca as either CaCO3 or 2% milk to the oral Mn essentially blocked the plasma uptake of Mn. No significant differences were found between the source of Ca in its inhibitory effect. Plasma Ca uptake was lower when Mn was simultaneously administered, but the results were not significantly different. Ionized levels of plasma Ca did not change significantly. The addition of Cu to the Mn load decreased the area under the curve for plasma Mn by about half, but it was not significantly different in the four subjects. In contrast, the addition of Zn to the Mn produced a significant increase in plasma Mn. Phosphorus has no influence on plasma uptake of Mn. These results indicate that the plasma uptake of Mn is greatly reduced by concomitant ingestion of Ca but may be increased by an oral load of Zn.