A comparison of IgE recognition by cognate receptors expressed on the C2 canine mastocytoma cell line with analogous events in a rat basophilic leukemia cell line transfected with the alpha-chain subunit of the canine high-affinity IgE receptor using flow cytometry show that canine IgE recognizes the alpha-chain of its cognate receptor on both cell lines. Our study confirms the expression of functional IgE receptors in both cell lines, but receptor-mediated signaling in the C2 line only supports the early stages of downstream signaling as shown by the phosphorylation of the gamma-chain and the failure to effect the phosphorylation of Syk. In contrast RBL-2H3 cells respond to sensitization with IgE and challenge with cognate antigen with tyrosine phosphorylation of the gamma-subunits of the receptor complex followed by downstream phosphorylation of Syk and Ca(2+) mobilization, culminating in beta-hexosaminidase release. We propose that the identification of the precise signaling defect in C2 cells will yield useful information regarding the pathway leading to mast cell exocytosis and facilitate the restoration of the complete signaling cascade through complementation of the missing/defective signal transducer since signaling events downstream of Ca(2+) mobilization are intact as demonstrated by beta-hexosaminidase release following non-immunologic stimulation with the calcium ionophore, A23187.