We have developed a simple, highly efficient, and rapid technique for labeling a commercially available human serum albumin microsphere preparation with 68Ga for use in positron tomography. An isotonic acetate buffer system is use. Maximum 68Ga labeling is obtained at pH 4.8 (approximately 97%) but deviations from that pH (e.g., 0.5 of a pH unit) produce only minor decreases in binding efficiency. The preparation is stable on standing over a period of 4 h, and in vivo studies in the rat and dog show it to be specific for the lungs. Little change occurs in the tissue distribution of 68Ga over a period of approximately 1 h.