In his previous report the author stated that Ascaris lumbricoides could be divided serologically into two groups, A ⁺ and A ⁻, by the presence or absence of A(Ⅳ) substance, and that the majority of the round worms parasitic in the persons whose blood types are A, AB or B belong to A ⁺ group, whereas those worms parasitie in O-type persons usually belong to A ⁻ group. In order to corroborate this statement experimental human infections were carried out in the present study. The eggs from two females of A ⁺ and A ⁻ type respectively were cultured to infective stage and swallowed, 10 to 16 each, by 10 volunteers of both sexes whose blood types are known a priori. The adult worms, 20 in all, recovered from the volunteers by administration of anthelmintica 3 months later were subjected to the inhibition tests of anti-A(Ⅳ) agglutinins for determination of their types. 1) Out of 11 worms recovered from three A and B blood type volunteers who swallowed a total of 36 eggs from A ⁺ type adult worm, 10 belonged to A ⁺ type and 1 to A ⁻ type. 2) Two O-type volunteers who swallowed a total of 29 eggs from A ⁺ type adult worm passed only one male worm each which proved to belong to A ⁻ type. This indicates that A ⁺ type adult worm produces A ⁻ type larvae besides A ⁺ type larvae. 3) One A blood type volunteer and another B blood type volunteer who swallowed respectively 12 and 15 eggs from A ⁻ type adult worm were not infected. 4) One A blood type volunteer, to whom 13 eggs from A ⁻ type worm were given, passed two worms of A ⁺ and A ⁻ type respectively, indicating that A ⁻ type adult produces larvae of both types as in A ⁺ type adult. 5) Two O-type volunteers who swallowed a total of 24 eggs from A ⁻ type worm discharged 5 worms, of which 4 were of A ⁻ type. From the above mentioned results, though complicated by the fact that one type worm, may produce larvae of the other type, it may be safely assumed in connection with the first report that the type substances, especially AIV substance, of Ascaris lumbricoides have a certain correlation with the blood types of the hosts in building up Ascaris infection.