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1. Lines were selected for divergence in yolk‐cholesterol concentration for three and four generations in the Cornell Control (Leghorn) and Athens‐Canadian (AC) (meat‐type) randombred populations respectively. Selection was based on both individual and family records.
2. Cholesterol values were obtained from two consecutively laid eggs when birds were 28 weeks of age. The yolks of eggs from individual hens were pooled and analysed for cholesterol content by the colorimetric assay of Zlatkis et al. (1953) on the “fat extract” obtained by a modification of the procedure of Folch et al. (1956).
3. The largest differences between high and low lines (1.lb27 and 1.lb38 mg cholesterol/g yolk) were observed in the last generation of selection. There was evidence of a lack of response in the low lines. Realised herita‐bility estimates obtained by dividing the difference between high and low lines by half the cumulative selection differential ranged from 0.lb11 to 0.lb25 in the AC population and from 0.lb21 to 0.lb25 in the Leghorn population. Realised estimates on a within‐line basis suggest that selection for yolk cholesterol is effective only in the upward direction.
