The biodistribution of 75Se and 125I were compared in nude mice bearing melanomas and injected simultaneously with biosynthetically-labeled 75Se-MoAbs and radioiodinated 125I-MoAbs from the same anti-melanoma hybridoma. The 75Se dose recovery in the tumor (day 10) was twice that for 125I, but the tumor to normal tissue ratios were identical. It is concluded that biosynthetically-labeled MoAbs survive better in vivo, while the tumor to normal tissue ratios are governed by the excretion rate of labeled MoAb catabolytic products.