The function of the hand depends to a considerable extent on the function of the metacarpo-phalangeal (MCP) joints. In the present work two methods were used to assess the mobility of these joints and measure their range. In 108 normal men and 120 normal women aged 20–80 years the angle between the third (middle) and fourth (ring) fingers was measured in both hands, with the MCP joints at 90 degrees flexion and with standardized abducting forces applied to the fingers. In 47 of the subjects standardized axial traction was applied across the MCP joint of the third finger and the joint space was measured from radiographs. Results showed that abduction of the flexed fingers varied between individuals, but did not change appreciably with age. The degree of abduction increased in proportion to the force applied, and visco-elastic behaviour could be demonstrated. Larger variations were found between individuals when the MCP gap was measured in traction; otherwise results were similar. By both methods females were significantly more flexible than males. The dominant hand was less flexible than the non-dominant. The effects of other factors on the mobility of MCP joints are described and compared with previous work.