Fragile X is the most common inherited cause of intellectual disability and is frequently associated with autism. The syndrome is due to mutations of the FMR1 gene that result in the absence of fragile X mental retardation protein (FMRP). We have developed a rapid, highly sensitive method for quantifying FMRP from dried blood spots and lymphocytes. This assay uses two new antibodies, a bacterially expressed abbreviated FMRP standard, and a Luminex platform to quantify FMRP. The assay readily distinguished between samples from males with fragile X full mutations and samples from normal males. It also differentiated mosaic from nonmosaic full-mutation male samples. This assay, because of its methodology and minimal cost, could be the basis for newborn or population screening.