Predicted and realized responses in a single generation of mass selection for an index and for its component traits were compared. The index included the log transformed traits determining egg mass in chickens to 40 weeks of age (days tested from sexual maturity, egg weight, rate of lay). The index was restricted to allow no increase in log days tested. Other traits measured were egg mass, age at first egg, egg weight, rate of lay, number of eggs and body weight. When averaged over replicates, realized and predicted responses were in close agreement for index values and for the component traits. Significant corresponding correlated responses were obtained for egg mass and weight. The restricted trait, log days tested, and the correlated trait age at first egg did not change. Egg mass was increased solely through change in egg weight.