The present invention relates to a rabbit monoclonal antibody that binds to human Id3 protein and/or mouse Id3 protein with high specificity and high affinity. The antibody has a binding constant, measured with respect to human Id3 protein or mouse Id3, of greater than 1.times.10.sup.8/molar. The antibody has no substantial cross-reactivity to other family Id proteins such as Id1, Id2, or Id4, or other endogenous proteins present in the cells that express Id3 protein. The specificity and high affinity of the rabbit monoclonal antibodies of the present invention allows sensitive and specific detection and/or quantitation of human or mouse Id3 protein in biological samples. The antibodies are useful in immunochemical-based assays such as ELISA, western blot, and immunohistochemical staining.