It is shown that the heterogametic sex of crosses between two highly inbred lines of animals provides a simple means to detect large effects of specific sex chromosomes on quantitative traits. Linear combinations of adjusted trait averages of matching types of crosses between three inbred lines of chickens were used to investigate sex chromosome effects. Evidence for a large influence of the W chromosome of one line on body weight was found. Estimated effects were between 2.4 and 7.2% of the corresponding phenotypic means. The autosomal genetic background exhibited a strong influence on the effect, thus suggesting a sizable amount of epistasis (interaction between sex chromosomes and genetic background).