Identifying the active ingredients of an intervention--intervention-specific components serving as key levers of change--is crucial for unpacking the intervention black box. Measures of intervention fidelity can be used to identify specific active ingredients, yet such applications are rare. We illustrate how fidelity measures can be used to identify program active ingredients in the context of a social-emotional learning intervention (Responsive Classroom). We applied one customary and two novel approaches to create indices of fidelity. In the customary approach, we averaged fidelity ratings across all core components. In the novel approaches, we computed fidelity indices for specific components by (a) averaging responses from like items and (b) deriving factor scores from a multitrait, multimethod factor analysis. We then tested indices in relation to achievement gains (N = 1,442). Indices derived using novel approaches explained more outcome variance than indices from the customary approach. Further, novel approaches revealed one component as a potential active ingredient. Discussion highlights strengths and limitations of the indices and implications for identifying program active ingredients.