This study employed a pretest-posttest control group design in a field setting with 38 supervisors and managers to test the effect of a theory-based mastery practice design for interpersonal skills training. The mastery practice protocol was drawn from recent research in cognitive and educational psychology on complex skill acquisition. Dependent measures included knowledge retention, behavioral skill demonstration, and far transfer to the workplace based on a multirater 360-degree survey instrument. In addition, qualitative data were collected using a semistructured interview process. Comparison of the mastery practice design to conventional behavior modeling workshop practice indicated improvements in retention and behavioral demonstration measures but failed to document any effect on transfer. Implications for future research and practice are discussed.