We have investigated the extent to which restriction fragment length polymorphism can be detected by two-dimensional electrophoresis of end-labeled genomic restriction fragments. Genomic DNA was digested with NotI and EcoRV and labeled at the NotI recognition site before first-dimension electrophoresis in disk gels. DNA in the disk gels was further digested in situ with HinfI prior to second-dimension electrophoresis, yielding patterns in which approximately 2000 end-labeled fragments were simultaneously visualized. On the basis of studies of 6 mother/father/child trios, a group of 184 fragments was organized into 85 polymorphic systems in which all allelic fragments were detectable in the 2-D patterns. Another 206 fragments varied as to their presence among individuals, but their relatedness to other fragments was not established. Our data indicate that a large number of DNA polymorphisms can be simultaneously scored in 2-D separations of genomic DNA fragments.