We report the implementation and exploitation of fluorescence polarization measurements, in the form of anisotropy fluorescence lifetime imaging microscopy (rFLIM) and energy migration Förster resonance energy transfer (emFRET) modalities, for wide-field, confocal laser-scanning microscopy and flow cytometry of cells. These methods permit the assessment of rotational motion, association and proximity of cellular proteins in vivo. They are particularly applicable to probes generated by fusions of visible fluorescence proteins, as exemplified by studies of the erbB receptor tyrosine kinases involved in growth-factor-mediated signal transduction.