Mycobacter avium complex serovars 4 and 20 were cultured in the presence of [3H] fucose, [3H]-methionine, and [3H]-mannose to specifically radiolabel the oligosaccharide of the glycopeptidolipid (GPL) antigens. Distribution of radioactivity in lipid was determined by thin-layer chromatographic methods. Examination of acid hydrolysates from radiolabeled antigens revealed that [3H]-methionine incorporated into methylated sugars in polar and apolar GPL components, whereas [3H]-mannose incorporated exclusively into the oligosaccharide of polar GPL antigens. Least incorporation of radiolabel into antigens was observed with [3H]-fucose. Use of radiolabeled serovar 4 antigens in macrophage uptake studies revealed maximum uptake to be slightly above 250 gg/ 3.2 x 105 cells. Timed experiments demonstrated that GPL antigens were relatively inert to degradation by resident peritoneal macrophages.