The effect of yeast culture (Saccharomyces cerevisiae) supplementation on laying hen diets was tested against flavomycin supplementation during 12 weeks, using 112 Brown Bovans laying hens, 52 week-old, divided into 7 equal groups fed on a basal diet containing 18.8% crude protein and 2810 kcal/kg ME (metabolizable energy) feed. Three groups were supplemented with 3 different levels of commercial yeast culture (1, 2 and 3 kg/ton) and 3 groups were supplemented with 3 different levels of flavomycin (50,75 and 100 mg/ton). ncreased levels of yeast culture supplementation significantly decreased hen-day egg production (p<0.05). Flavomycin supplementation showed the same result with the exception of the group that received 75 mg/ton which showed significant (p<0.05) increase in hen-day egg production. Egg mass was significantly increased (p<0.05) for the group receiving 75 mg flavomycin and significantly decreased in the group receiving 3 kgyeast culture. Both yeast culture and flavomycin supplementation significantly lowered (P<0.05) feed intake as mg/hen/day. Feed conversion was improved significantly (p<0.05) by the addition of 2 kgyeast culture and 75 mg flavomycin per ton feed. The addition of 50 mg flavomycin improved yolk percentage significantly (P<0.05). The blood constituents showed no significant differences among treatments for TP, ALand GL (P>0.05) AST, ALT and Glu values showed an increasing trend, with the highest significant values (P<0.05) of AST and Glu for the supplementation level of 100 mg flavomycin. Higher level of flavomycin (100 mg/ton) had adverse effects on blood plasma biochemical profile, but had no significant effects on egg production (P>0.05). پرونده مقاله