Abstract: A radiation counter and method for measuring lean muscle mass. The invention is small, lightweight, economical and easily transportable because it eliminates the use of large, heavy room-sized and source-size shielding to block ambient radiation. The radiation counter is particularly suited for measuring gamma radiation from K.sup.40 out-of-doors since the primary outdoor sources of K.sup.40 are animals and the ground. The radiation counter comprises a frame for supporting a radioactive source and a radiation detection unit ("RDU"). The RDU comprises a scintillation detector and a lead shield partially enclosing the scintillation detector. An aperture is formed in the top side of the shielding to expose a portion of the scintillation detector and thus permit the detection of radiation in the desired energy level. The radiation counter is positioned with respect to the source so as to block the penetration of background radiation and eliminate the use of a large, heavy shield.