As part of the Harvard Cancer Prevention Program Project, we sought to address disparities reflected in social class and race/ethnicity by developing and testing a behavioral intervention model that targeted fruit and vegetable consumption, red meat consumption, multivitamin intake, and physical activity in working-class, multiethnic populations.This paper examined the associations between change in leisure-time physical activity and individual and social contextual factors in participants employed in small businesses (n = 850) at both baseline and at 18-month final.In bivariate analyses, age, language acculturation, social ties, and workplace social capital were significantly associated with physical activity at final. In multivariable analyses, being younger and having high language acculturation were significantly associated with greater leisure-time physical activity at final; high workplace social capital was significantly associated with a decline in physical activity at final.These findings have implications for understanding factors that are integral to promoting change in physical activity among working-class, multiethnic populations.