The patterns of isolated metacarpophalangeal-joint flexion and isolated interphalangeal-joint extension movements in eighty fingers with intact extrinsic and lumbrical muscles but no interossei (so-called interosseus-minus fingers) were studied. The pattern of metacarpophalangeal flexion resembled that seen in fingers with paralysis of all intrinsic muscles. From this it was inferred that in the interosseus-minus finger the lumbrical muscles are inactive during this movement. The pattern of isolated interphalangeal extension in the interosseus-minus fingers showed an improvement over that in totally intrinsic-minus fingers, suggesting that the lumbrical muscles reduce the extending forces at the metacarpophalangeal joint to some extent. Active participation of the interosseus muscles appears to be necessary for carrying out the movements of isolated metacarpophalangeal flexion and interphalangeal extension.