Cultured human B-lymphoid cells WIL-2 were mutated with ethylmethane sulfonate and selected for resistance to tunicamycin, an antibiotic that selectively inhibits N-linked glycosylation. Ultrastructural analysis of five isolated tunicamycin-resistant mutants (TMR) showed changes in surface microvilli, dilation of the endoplasmic reticulum, and an increase in surface myelin figures. Cytofluorometric analysis of TMR cells incubated with anti-HLA monoclonal antibodies showed a normal density of HLA-A, B antigens and an increased density of Ia antigens. The properties of these TMR mutants have remained stable for at least 18 months. These TMR cells may serve as a useful model to study the biochemical events in the processing and expression of Ia antigens.