The early 1950s saw dramatic changes in the world of U.S. science fiction publishing. At the start of 1949, all but one of the major magazines in the field were in pulp format; by the end of 1955, all had either ceased publication or switched to digest format. This change was largely the work of the distributors, such as American News Company, who refused to carry the pulp magazines since they were no longer profitable; the loss of profitability was in turn associated with the rise in mass-market science fiction publishing, with paperback publishers such as Ace Books and Ballantine Books becoming established. Along with the increase in science fiction in book form came a flood of new U.S. magazines: from a low of eight active magazines in 1946, the field expanded to twenty in 1950, and a further twenty-two had commenced publication by 1954.