Preliminary studies were performed on heterophile antibodies by rabbit and sheep hemagglutination tests in various oral mucous diseases. On rabbit hemagglutination test, most sera from normal healthy persons showed titers from 256 to 512,while the test on sheep revealed titers from 64 to 128. These titers roughly paralleled each other, although the latter showed a titer of 2^2 lower than the level of the former. On the other hand, the titer of patients with lichen planus or oral candidiasis on these tests showed a level close to normal. Patients with aphthous stomatitis or recurrent aphthous ulcer revealed a fairly lower than normal titer on both tests. From the results of an absorption test using sediment of kidney or erythrocyte corpuscle of various species, it was suggested that the heterophile antibody on the rabbit hemagglutination test did not belong to neither the Forssman, Paul-Bunnell (P-B) or serum sickness (Hanganutziu-Deicher; H-D) types.