This article investigates the effect of union organizing as a mobilizing strategy on the collectivism of union members. We examine the impact of a worker's social identification with fellow members and the transformational leadership qualities of the local union representative. We employ regression analysis with tests of mediation to analyse the survey responses of c. 1,000 rank and file members of a major professional union, collected in July 2004 during a mobilization campaign. Social identification and transformational leadership were associated with members' union loyalty and willingness to work for the union. Social identification acted as a mediating variable in both cases.