This paper reported the identification of a novel optical signature for epidermal growth factor (EGF) receptor signaling in human epidermoid carcinoma A431 cells mediated by EGF. The optical signature was based on dynamic mass redistribution (DMR) in living cells triggered by EGFR activation, as monitored in real time with resonant waveguide grating biosensors. Analysis of the modulation of the EGF-induced DMR signals by a variety of known modulators provided links of various targets to distinct steps in the cellular responses. Results showed that the dynamic mass redistribution in quiescent A431 cells mediated by EGF required EGFR tyrosine kinase activity, actin polymerization, and dynamin and mainly proceeded through MEK. The DMR signals obtained serve as integrated signatures for interaction networks in the EGFR signaling.