Two genetic systems in the mouse have been studied -- the Tla-H(Tla) system and the W system. Tla-H(Tla) System: Several histocompatibility and serologically defined loci have been found in the Tla region (defined as that segment of the 17th chromosome starting immediately to the right of H-2D and ending at some point distal to Tla) of the mouse. The most interesting of these is the Qa-2 locus which determines two lymphoid antigens, Qa.2 and Qa.3, which define subpopulations of lymphocytes. Qa.2 is similar in its molecular weight to H-2 antigens: 43,000 daltons with a beta-2 microglobulin subcomponent. W System: Using a cytotoxicity test on liver cells from 14-day-old mouse fetuses, we have determined that normal and mutant (W/W) erythroid cells differ in their display of stage-specific erythroid antigens detectable by two rat antisera. The mutant (W/W) cells prematurely lose an "early" antigen(s) recognized by a rat antierythroblast serum and gain a "late" antigen recognized by a rat anti-normal adult mouse red blood cell serum. BIBLIOGRAPHIC REFERENCES: Flaherty, L., Stanton, T. and Boyse, E. A. 1977. Contamination of Ia antiserum, A.TL anti A.TH, with antibodies related to the Tla region. Immunogenetics, in press. Michaelson, J., Flaherty, L., Vitetta, E. and Poulik, M.D. 1977. Molecular similarities between the Qa-2 alloantigen and other gene products of the 17th chromosome of the mouse. J. Exp. Med., in press.