Biological age was assessed in a methodological field study in two unselected Finnish populations which comprised 460 males aged 25–27, 35–37, 45–47 and 55–57 years. The test battery consisted of six tests which had the following correlations to chronological age: threshold for vibratory stimuli in ankle (0.624), digit symbol test (––0.588), vital capacity (––0.594), audiometry at 4,000 cps (0.563), simple reaction time (0.329), and index of cortical thickness of metacarpal bone (––0.299). Multiple correlations to chronological age ranged between 0.744 and 0.831, depending on the variables included in the index. The length of education, occupation and district of living explained partly the variation i