The present study was undertaken to reveal the magnesium distribution in human bone. Sixty human ribs, obtained from subjects aged 10-80 years of age, were used. Transverse sections were prepared from the middle region of the human ribs. Adjacent sections were ground to a thickness of about 1000 microns. One section was used for magnesium determination by atomic absorption spectrophotometry, and the other was used for analysis with X-ray microanalysis. Thirty micron thick samples were abraded continuously from the periosteal and the endosteal surfaces by abrasive microsampling, as previously described by Weatherell et al. Results showed that magnesium concentrations were higher in both the periosteal and endosteal surfaces and did not change with age in general, although it tended to be higher among teenagers and lower over 80 years old.