An absence of galactose on the N-linked oligosaccharides of immunoglobulin G (IgG) has been shown to affect the functional activity of the antibody molecule. In patients with rheumatoid arthritis there is an increased proportion of IgG which lacks galactose and correspondingly lower levels of β1, 4-galactosyltransferase (β4Gal-T) activity. The recent demonstration of several expressed β4Gal-T genes in man raises the possibility that the enzyme responsible for the decreased IgG galactose is not the “classical" β4Gal-T (β4Gal-T1). To directly address the question of whether reduced β4Gal-T1 would lead to reduced IgG galactose, the level of β4Gal-T1 in a human IgG-secreting B cell line was specifically altered using stable transfection with sense (SpcDNA3-Gal-T1) or antisense (ASpcDNA3-Gal-T1) human β4Gal-T1 cDNA. SpcDNA3-Gal-T1 B cell transfectants expressed up to a 2.5-fold higher level of β4Gal-T enzyme activity for the exogenous neoglycoconjugate acceptor GlcNAc-pITC-BSA than did ASpcDNA3-Gal-T1 transfectants. Flow cytometric analysis with Ricinus communis agglutinin I (RCAI) revealed an overall greater number of Galβ1, 4GlcNAc structures in the fixed and permeabilized SpcDNA3-Gal-T1 B cell transfectants compared with the ASpcDNA3-Gal-T1 transfectants. Moreover, there was increased galactosylation of IgG secreted from the SpcDNA3-Gal-T1 transfectants relative to the ASpcDNA3-Gal-T1 B cell transfectants. Alteration of the level of the “classical" β4Gal-T (β4Gal-T1) in B cells therefore affects IgG glycosylation.