A current trend in kindness research is to assess the effect of being kind on participants' well-being. To do this, participants are asked to complete a series of kind acts and the corresponding impact on their well-being is measured. As participation in school-based interventions can vary, the aim of the current study was to assess the extent of adolescents' engagement in a kindness intervention and the resultant effect on their well-being. An intervention study was conducted in which 383 sixth through eighth graders planned and completed three kind acts per week for four weeks, with pre- and post-test assessments of well-being administered. Adolescents' acts of kindness reflected the themes of helping with chores, being respectful, complimenting/ encouraging others, and giving objects or money. No significant differences between control and intervention groups at post-test on any well-being measures were found, after controlling for pre-test scores. However, upon analysis of participants' engagement in the intervention (intervention uptake), it was determined that half of the participants (n=87) implemented less than 60% of their kindness intervention. Participants were thus clustered into three groups: zero, low, and high implementers. ANCOVAs revealed that high implementers had the lowest self-reported negative affect and highest self-reported kindness to others. Implications for adolescent prosocial development are discussed.