A project designed to foster the full and fair participation of girls in high-school science classes addressed obstacles, both perceived and actual, to equal participation. In order to modify existing classroom techniques and environments, a Teacher Intervention Program was designed. By means of a workshop and periodic personal communications, teachers were sensitized to the importance of a stimulating, gender-free learning environment. In addition, they were presented with a variety of methods and materials which had been shown to encourage girls in science. Twelve teachers, who were selected randomly, taught in diverse communities throughout one Midwestern state. The subjects tested were students in 24 general biology classes taught by the 12 teachers. Although both qualitative and quantitative measures were used during the research, only the quantitative results are discussed in this paper. Using ANOVA's, treatment group by student sex, a comparison of the mean scores was made for all students, as well as for all females and for all males. The results indicated that the experimental group, compared to the control group, had significantly higher mean scores on tests of attitudes toward science, perceptions of science, extracurricular science activities, and interest in a science-related career.