SUMMARYOne group of embryonated chicken eggs was inoculated with sterile mycoplasma broth and another with a broth culture of Mycoplasma synoviae. Each group was subdivided into two and the chicks were hatched in four isolation pens. At 1 day of age one uninfected and one M. synoviae infected group were inoculated with a reovirus known to cause tenosynovitis. Birds were closely observed, and some were killed for sampling as chicks (1-5 weeks) and the remainder as adults (25-30 weeks). Clinical signs related to both organisms were seen in the dual-infected chicks and were more pronounced than in either of the corresponding single infections. There were significant weight differences between the infected and control chicks, except in single M. synoviae infection, and the mean weight of the birds with mixed infection was less than those with either single infection. Gross and histological lesions were observed in the hocks and livers of chicks in each infected group although with a higher incidence and severity in the dual-infection. Microscopic lesions were seen in the hearts of some birds in all three groups. Leg lesions associated with the reovirus infection persisted to maturity and another observation in mature, virus-infected birds was impaired egg production and fertility.