Parabiotic compatibility is primarily dependent on identical alleles at the H-2 locus. “Bilateral” difference at the H-2 locus—each partner having an H-2 allele not carried by the other—results in separation of partners during or after the 2d week of union. Separation is preceded by cessation of dye transmission (Evans Blue and Hypaque) from the 6th day on. Persistence of union beyond the usual time of separation, “false union,” is merely passive and not accompanied by dye transmission. “Unilateral” difference at the H-2 locus, one partner having an H-2 allele not carried by the other, as in many “pure” F1 hybrid unions, usually results in “parabiotic intoxication” of the partner carrying the foreign allele. Death commonly occurs during the latter half of the 2d week or the first half of the 3d week. Dye transmission ceases early during the 2d week. Dissimilarity in sex, at the H-1 or the H-3 locus, does not result in separation or intoxication. In all groups compatible at the H-2 locus Hypaque transmission has been observed for at least 2 months after union. The subject of parabiotic intoxication is discussed.