Since 1948, avian unknown diarrhea resembling the so-called pullet disease in America has occured all over Japan.The egg production of laying flocks ceases within a week and does not return to a profitable level until about one month later.The course of the disease extends over a period of from one to two weeks, and terminates in a high percentage of apparent recovery. In laying birds, very small egg (Fig. 1) or soft egg production is found.The blood picture is a relative monocytosis, which averages over 20 per cent, in comparison with the normal of from 5-12 per cent. In artificially affected birds, monocytosis occurs parallel with the symptom, and returns to a normal level when the symptoms disappear.Histological findings in organs (reported in the follow-ing report) are very similar to that of pullet decease.Artificial infection was successfully used for the first. time by the authors, by contact with the affected birds, per os inoculation with the intestinal contents, i. c. inoculation with brain, i. v. inoculation with the blood of the affected bird.Healthy birds were affected with per os or i. v. inoculation with the filtrate through Chamberland L2 or L3 of the intestinal contents of the bird affected with unknown diarrhea (Chart 6, 7, 8)The authors have succeded in isolating two strains of a filtrable agent from the intestinal contents of the bird affected in acute form. The virus can be cultivated in the allantoic cavity of an 8 to 10-day-old chick embryo, where it produces the death of the embryo almost within 36-72 hours after inoculation (Chart 9, 10). Hemorrhage is often found around the eye-lids (Fig. 2, 3). In rare cases, some embryos die after about a week or survive after inoculation. Such embryos are very small in comparison with the normal embryo (Fig. 2).Healthy birds are affected with the freshly isolated virus, showing typical symptoms and monocytosis after 5-6 days after per os or i. v. inoculotion (Chart 11, 12).From above-mentioned results, a causative agent of avian unknown diarrhea (similar to so-called pullet disease) is a virus, here to fore unreported.Neutralization test is positive between the freshly isolated virus and the serum of recovered dirds. Recovered bird have resistance to challenge, while the birds in control were affected typically with the challenge.So avian unknown diarrhea (similar to pullet disease) should be designated as AVIAN INFECTIOUS DIARRHEA (Diarrhoea infectiosa avium)