An experiment was carried out to evaluate a sample of hydrocarbon-grown yeast as a protein source for replacement pullets and laying hens. It was designed as a 2 x 3 x 3 x 4 factorial experiment in which two egg-laying strains (Thornber 808 and 909) were given diets containing 0, 5 or 10% yeast during the starter (0-6 weeks) and grower (7-18 weeks) stages and 0, 5, 7.5 or 10% yeast during the laying stage (20-68 weeks). A total of 720 chicks of each strain were reared in 36 floor pens until 18 weeks of age, 432 of each strain were then transferred to individual layers' cages and food intake and egg production were recorded for 12, 4-week periods from 20 weeks of age. Birds given the 5 and 10 % yeast diets were significantly lighter at 6 weeks of age and those given the 10% yeast diet between 7 and 18 weeks were significantly lighter at 18 weeks than birds given the respective control diets. Food conversion efficiency and mortality were not significantly affected by dietary treatment. None of the rearing and growing treatments significantly affected subsequent egg production. Light-and medium-hybrids given the layers' diet containing 10 % yeast produced on average four and nine eggs, respectively, fewer than birds given any of the other diets. The inclusion of yeast in the diet reduced the food intake of the medium-hybrids by about 3% but did not affect the food intake of the lighthybrid. Food conversion efficiency was not significantly affected by any of the laying treatments. Medium-hybrids given the yeast-containing diets were significantly lighter at 68 weeks of age than birds of the same strain given the control diet. Since the layers' diets, formulated to be no more than adequate in protein and amino acid content, allowed a high level of production from the light-hybrid it is concluded that the hydrocarbon-grown yeast is no less adequate nutritionally than the proteins it replaced. The poorer performance of the medium-hybrid when given the yeastcontaining diets may have been caused by a strain difference in food preferences or in nutrient requirements.
