Divergent selection for attachment strength between the shell membrane and the calcium shell was performed in a White Leghorn strain. Multivariate analysis was used to estimate genetic parameters for shell membrane measurements and shell thickness. The aim was to investigate the possibility of improving shell strength in laying hens by selecting for increased attachment strength. 2. A significant direct selection response for attachment strength was achieved. There was a favourable correlated selection differential in the frequency of cracked eggs which resulted in a nonsignificant but favourable, correlated response. Selection for a strong attachment resulted in a thinner egg shell, and vice versa. Heritabilities were all relatively high (0.30 to 0.70). Most of the genetic correlations were in agreement with the achieved correlated responses. 3. Two factors that probably contributed to the responses in egg shell thickness were an unfavourable genetic correlation between attachment strength and shell thickness, and natural selection against changes in attachment strength, in the form of increased mortality during incubation and hatching. Problems connected with breeding for increased shell strength are discussed.
