Histopathological lesions were studied in chickens with maternal antibody against MD virus at 14-day intervals after their exposure by contact to HPRS-16 of MD virus in the first-day of life. The first lesions occured 14 days after infection in the liver, kidney, heart, brain and the sciatic nerves. Relatively, the most expressive changes were observed between 56th and 70th day after infection. Lesions were characterized as light to heavy infiltrations formed by immature and mature pleomorphic mononuclear cells. Among these cells myeloid type cells occured too. In some cases expressive tumorous lesions with intensive mitosis were observed mainly in the liver. Intranuclear inclusion bodies were also observed mainly among the proliferating epithelial cells of the proventriculus. 14-days after infection changes in the brain were classified as nonpurulenta encephalitis. Gross lesions characteristic of MD occured the most frequently in the gonads, liver and the kidney. In one case 56 days after infection herpesvirus-like particles were observed by electron microscopy in the tumorous gonads and liver. They measured about 200 to 260 nm in diameter. Degraded and/or aberrant incompleted virus-like particles occured the most often and completed (enveloped) ones with electrondense nucleoids were observed occasionally. It was concluded that MD in chickens with materanal antibody against MD virus shows monophasis progress.