To identify factors associated with participation in the 1986 Ontario, Canada doctors' strike, we surveyed 1,028 physicians; 69 percent responded, of whom 42 percent participated in the strike. Risk factors for participation included income greater than $135,000, being a surgeon or gynecologist, having previously "opted out" of the Ontario Health Insurance Plan, being professionally dissatisfied, being politically conservative, favoring political activism by physicians, holding a positive view of the social consequences of extrabilling, and perceiving family, associates, patients and the public to favor the strike. Eighty percent of strikers, but 32 percent of non-strikers, met criteria we established for four strike-prone groups: the "economically rational," the "ideologically committed," the "professionally disaffected," and the "socially malleable." Respondents belonging to one or more of these groups were much more likely to have participated in the strike (64 percent vs 17 percent). Strategies to deal with physician militancy should address the multiplicity of motives that appeared to have influenced doctors in Ontario.