This study examined the effects of the Local Systemic Change Through Teacher Enhancement Initiative (LSC) on 1,596 in-service high school mathematics teachers' knowledge. Because data are clustered by schools, a hierarchical linear model was calculated. It was found that five of the characteristics of the professional development, time to reflect, support to implement, hours attended, assistance to implement from LSC, and participation in message boards, had statistically significant effects on increasing teacher knowledge, but time to work with other teachers, race, educational background, and years taught did not have a statistically significant effect. [For the complete proceedings, see ED606531.]