Objective.-To assess expression and function of cell-surface IgE receptors on the canine mastocytoma cell line C2 maintained in continuous culture. Sample Population.-C2 cells maintained in medium lacking IgE for up to 10 passages before being stored at -80 C. Procedure.-Cells were thawed, cultured in medium without IgE for 1 to 3 passages, sensitized for 7 days with IgE-rich serum from dogs naturally sensitized to Ascaris suum, and stimulated with antigen Asc S1 from A suum, goat polyclonal anti-canine IgE, or calcium ionophore and phorbol myristate acetate (PMA). Percentage of intracellular β-hexosaminidase released and concentration of tumor necrosis factor-α (TNF-α) synthesized after stimulation were determined. Expression of cell-surface IgE receptors was assessed by use of a flow cytometry. Results.-Immunologic stimulation (antigen or anti-IgE) failed to induce release or synthesis of detectable amounts of β-hexosaminidase or TNF-α. In contrast, nonimmunologic stimulation (calcium ionophore and PMA) led to release of β-hexosaminidase (mean ± SEM maximum release, 23.95 ± 1.96%) and synthesis of TNF-α (maximum concentration, 34.34 ± 2.34 pg/106 cells). As revealed by use of flow cytometry, C2 cells expressed surface IgE receptors that bound canine IgE in vitro. Conclusions.-Continuous culture of the canine mastocytoma cell line C2 in medium without exogenous IgE or cytokines and other growth factors resulted in cell-surface expression of nonfunctional IgE receptors. However, C2 cells maintained in continuous culture may still be a useful tool for the evaluation of mast cell responses to nonimmunologic stimulation and IgE receptor differentiation and maturity.