We have constructed chimeric receptors, combining murine IgM and the cytoplasmic portion of human epidermal growth factor receptor (EGFR), with the aim of developing a novel immunosensor with antigen-dependent phosphorylation activity. When intact IgM was used, the chimeric receptor showed both antigen binding and protein tyrosine kinase activity, but the kinase activity was constitutive and independent of antigen binding. However, with IgM lacking the CH2 domain, the autophosphorylation activity increased with increasing concentrations of anti-IgM or hapten-BSA conjugate. Monovalent hapten could not induce phosphorylation but inhibited stimulation by hapten-conjugated BSA.