A hundrerl and thirty sroDific thogen-fiee (SPP), 4-week-oH chickens were dividnd into seven group. They were hif th (1) IBDV (2) NOV velogenic stain (3) NOV mesogenic stain (4) NOV lentogenic stain (5) both IBDV and NOV velogenic stain sirnulnsc,us1y,and( \\\ilx1v fdlowal by NOV 3 days ncculation. The ‘7th group was used for control, The lyinphoid organs including Smninary the burn ofFairicius. spleen, cecal tonsil and thyrnus, were damaged by the infection of IBDV alone or NOV alone or their co-infadolL Encephalitis was sen in all chickens of the 3rd group and afew chickens of the 4th group but not sen in other groups. The result suggested that the lesions of the lyrnphoid organs had noel in the outbreak of Newcastle disease in the middle and southern Taiwan since February, 1984, might ke causal by NOV alone or both NOV and IBDV.