B16 melanotic tumors in various organs of mice were labeled with either [14C- or [3H]L-fucose or D-glucosamine. Glycopeptides derived from the membrane glycoproteins of these tumors were compared with those of normal tissues by double-label elution patterns from Sephadex G-50 columns. A marked increase of sialic acid-rich, fucose-containing glycopeptides (PEAK A) was found in the glycoproteins of the surface and internal membranes of melanotic cells. The glycopeptides from the melanoma cells could be reduced in size by treatment with neuraminidase. Comparison of the glycopeptide patterns of melanoma cells grown in culture and in mice revealed a greater complexity in in vivo material. Virtually all of the glycopeptides from melanoma cells grown in culture were of the larger type (Peak A) that correlates well with the malignant state. Comparison of two lines of B16 melanoma cells with greatly differing abilities to form tumors in lung revealed no significant, reproducible differences in their glycopeptide patterns.