Chronic exposure to high levels of manganese (Mn) leads a pronounced and debilitating disorder known as manganism. Research on the toxic manifestation of manganese have focused primarily on its neurological effects because exposure to high levels of the metal produces a distinct and irreversible extrapyramidal dysfunction resembling the dystonic movements associated with Parkinson's physiological and biochemical systems in the body. The purpose of this study is to determine the effects of Mn on mineralization in primary rat calvarial cells. The experimental groups were in concentration of 0, 10, 30 and 60 micrometer. The results were as follows: 1. ALP activity was decreased in concentration of 30 and 60 micrometer (p<0.01). 2. Bone nodule formation was depressed in concentration of 30 and 60 micrometer at day 14 and 21 (p<0.01). 3. RT-PCR results showed an altered expression of bone matrix proteins. These result suggested that manganese might decrease or alter the expression of the osteoblast phenotype.