The level of β1-4 galactosyltransferase activity was examined in a number of spontaneously, chemically, or virally transformed murine tumor cell lines. Increased levels of enzyme activity were observed for the murine myeloma cell line K181 and in vivo MOPC 104E. The Maloney Sarcoma Virus (MSV) transformed T-cell lymphoma, YC-8, also demonstrated elevated levels of enzyme activity when compared to a second independently MSV transformed T stem-cell lymphoma, LSTRA. Cell surface immunofluorescence was also detected in YC-8 with a monoclonal antibody for galactosyltransferase. The introduction of galactosyltransferase specific substrates, both in vivo and in vitro, led to the retardation of growth in the cell lines K181, MOPC 104E, and YC-8, but not in the cell line LSTRA; this suggests the selective growth control of transformed cells demonstrating elevated levels of galactosyltransferase.