This study used data from 850 African Americans to test optimal matching theory (OMT). OMT predicts that 1) the most important dimensions of social support depend on the controllability of the behavior and 2) different network members often provide support across health behaviors. Data were gathered on social support source for physical activity, healthy diet, and colorectal cancer (CRC) screening; perceived levels of informational, instrumental, and emotional support specific to these behaviors; self-efficacy around the behaviors; and engagement in the behaviors. Within individuals, the primary support source varied considerably across the behaviors under consideration. Multivariate models regressing behaviors on dimension-specific support indicated partial support for OMT: Informational support was associated with a healthy diet and CRC screening; instrumental support was associated with a healthy diet and CRC screening; and emotional support was associated with CRC screening and, among women, physical activity. Implications of the findings are discussed in terms of developing more effective interventions.