Hormone transfer theory predicts that opposite-sex twins will be affected in utero by the hormones their twins produce. This study examines the prediction that opposite-sex dizygotic twins should show less stereotyped toy play than same-sex dizygotic twins should show less stereotyped toy play than same-sex dizygotic twins as a result of exposure to opposite-sex hormones in utero. Participants in this study included 32 male-female dizygotic twin pairs, 27 female-female pairs, and 24 male-male pairs. All ranged in age from 7 to 12 years and were primarily Caucasian and middle class. Differences in the amount of time children played with feminine, masculine, and neutral stereotyped toys during free play were examined. Results did not support hormone transfer theory. Girls played with feminine toys more than boys and boys played with masculine toys more than girls, but there were no significant differences between children with opposite- vs. same-sex twins.