An increasing number of genetically modified mouse mutants are employed in immunological research. Many experiments require the crossbreeding of various mouse lines and screening for the genetic background of the offspring. Analysis for various immunological phenotypes such as the MHC class I haplotype is usually performed by FACS analysis of peripheral blood lymphocytes. Here, we report a PCR based technique for the differentiation of the MHC class I H-2K(k) and the H-2K(b) haplotype. Advantages of this method are cost efficiency and rapidity in particular when multiple genetic variables such as the disruption of a gene, transgenesis or the MHC haplotype are to be tested.