To determine the distribution of beta-2 microglobulin (B2m) alleles in wild mice we have typed mice derived from natural populations in Europe, North Africa, South America, and East Asia. Mus musculus domesticus mice from Germany, France, Italy, and Peru were all B2m a as were most from the United Kingdom. M.m. musculus mice from Denmark and Czechoslovakia, several stocks of M.m. molossinus from Japan, and M.m. castaneus from China, Thailand, and the Philippines were of B2m b type. This is consistent with the notion that C57BL/6 may have obtained some of its genes, including B2m, from Eastern mice. A BglI restriction site characteristic of B2rn b was also found in mice from Czechoslovakia and Japan, confirming that B2rn b is a naturally occurring allele of B2m. A new type of/~2m (/~2 mwl) was found in four stocks of M. spretus from Portugal, Spain, and Morocco. This molecule differs in apparent size and charge from the a and b types./~2m w2 was found together with/32m a in one stock of M.m. domesticus (brevirostris) from Morocco./~2m w3 and j32 mw4 Were found in a few M.m. bactrianus from Pakistan. In all cases tested, these new/32m molecules associate with class I histocompatibility antigens.