It was later observed that the IB was common in semimature and lying flocks as well and caused a marked losses in egg production. Prior to 1956 avian infectious bronchitis was considered to be caused by a single antigenic type of virus (pathogenic Massachusetts M₄₁, and the Beaudette non pathogenic embryo lethal strain M, since that time a number of isolates have 42 been identified from outbreaks that are different antigenically from the original Massachusett type Winterfield et al., 1971 and Hopkins, 1974). Variant strains most frequently found to be involved in IB problem herds, belonged to two main groups namely (D₂₀₇ and D₂₁₂) which have poor immunogenic properties. A field isolate called D₂₇₄, however belonging to the D₂₀₇ serotype, proved to induce cross-protection with D₃₈₉₆ and to a lesser extent D₃₁₂₈. Also, in the D₂₁₂ group haemagglutinating strains were found, of which D₁₄₆₆ was found to be more immunogenic than D₂₁₂. Mycoplasma gallisepticum [MG] infected chickens have been reported to suffer from a drop in egg production [Carpenter et al., 1979; Carpenter et al., 1981; Lin and Kleven, 1982). MG infected hens have been observed to lay an average of 15.7 fever eggs/hens housed than MG free hen over a 45 week laying period [Carpenter et al., 1981). The aim of this work was to demonstrate and evaluate the role of IB variant strains either alone or combined with Mycopeasma gallisepticum on the egg production of layer breeder flocks.