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