Cells were treated with the conditioned medium from opposite cell types and immunoprecipitated with antibody to ErbB1 (A, E), ErbB2 (B, F), ErbB3 (C, G), ErbB4 (D, H) under co-immunoprecipitation conditions. Panels show representative blots for fibroblasts (A, B, C, D) and type II cells (E, F, G, H); illustrated results were consistently found in 3–5 independent experiments.