In Japan, not a few Japanese people may believe that blood group polymorphisms of the ABO system are related to personality differences. Blood-typing is regarded here as a lay people's personality theory. The aim of this study is to understand that how people use this theory and that what functions the theory Serves. Subjects are students of a university and those of a nursing-school. 7 hypotheses derived are bellow. 1) this theory is popular among these subjects, 2) most of subjects cannot tell whether this theory is valid or not, 3) when someone begins to talk about the topics, conversations would be maintained for a while, 4) this theory can tell about both individuals and personal relationships. and this theory is useful for both beginning conversations and self-presentation at interpersonal situations, 5) contents of this theory are made of generalizations, 6) guesses of others' blood types are influenced by both theory's criteria and impressions of others, 7) evaluations to minor blood groups (B and AB) are relatively negative. After discussing the validity of blood-typing as a personality theory, the implications of this study are discussed.