Over 50 lines of histocompatibility mutants have been or are being developed from H-mutations recovered in mice in this laboratory. These mutants, and new ones which will arise in the future, are valuable for a wide range of immunogenetic and immunobiologic research. We propose to establish and maintain mutant lines, to isolate and identify new mutations, to map the mutations insofar as may be practical, and to utilize the mutants in immunogenetic studies. We are currently working with 13 H-2 and 30 non-H-2 mutant alleles, with the remaining mutants yet to be classified. In addition to our own genetic analysis, the established mutant lines are also being studied through collaboration with other laboratories with respect to in vitro cell-mediated reactions, correlations between serology and histocompatibility, mapping of mutant genes within the H-2 complex, and alloantigen structure.