American unionization levels peaked at nearly 35% in 1945. This compared favorably with the West German peak of 36% in 1951, it should be noted, although the U.S. and Germany have obviously diverged on this dimension since the 1950s.
Steven Greenhouse, "Membership in Unions Drops Again," New York Times, 1/28/05. For the American peak: Paul Boyer, ed., The Oxford Companion to United States History (New York: Oxford University Press, 2001), 429; for the West German peak: Wendy Carlin, "West German Growth and Institutions, 1945-90," in Economic Growth in Europe Since 1945, ed. Nicholas Crafts and Gianni Toniolo (New York: Cambridge University Press, 1996), 467.