A lymphocytic tumor, 38C‐13, induced by the chemical carcinogen 7, 12‐di‐methylbenz(a)anthracene in C3H/eB mice and adapted to tissue culture, produces 7–8 S IgM with “core” carbohydrates (N‐acetylglucosamines, mannoses), but not “branch” carbohydrates (neuraminic acids, fucoses, galactoses) attached to the μ heavy, but not to the light chains. Turnover of the 7–8 S 38C‐13 IgM is slow (half disappearance time = 10–15 h). The IgM is released from the cells as 7–8 S IgM. The ratio of IgM synthesis to the synthesis of all cellular glycoproteins is 0.005–0.01. After comparison of these data with data obtained with normal B lymphocytes before and after mitogenic stimulation, we conclude that 38C‐13 tumor cells are transformed counterparts very near or within the population of small, mitogen‐sensitive, resting B lymphocytes.