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A cooperative study involving 280 crossbred pigs was conducted at three experiment stations (Illinois, Indiana and Kentucky) to evaluate the efficacy of the antibiotic, thiopeptin, as a growth promotant for growing pigs. At each station, two replications of barrows and two replications of gilts (four or five/pen), initially averaging 9.3 kg, were fed a fortified corn-soybean meal basal diet with 0, 5.5, 11, 22 or 44 ppm thiopeptin. The pigs averaged 41.4 kg at the end of the 56- to 57-d experimental period. Daily gain increased quandratically (505, 550, 565, 585, 590 g/d; P less than .001) and feed required per unit of gain decreased quadratically (2.32, 2.26, 2.29, 2.22, 2.22; P less than .15) with increasing levels of thiopeptin. Breakpoint analysis indicated that rate and efficiency of growth were maximized at the 14- and 22-ppm levels of thiopeptin, respectively. Averaged across all levels of thiopeptin, gain was increased by 13.4% and feed/gain by 3.2% in pigs fed the antibiotic. Responses to dietary treatment were similar at each station, with no evidence of a treatment X station interaction for gain (P = .35) or feed/gain (P = .80). Barrows and gilts performed similarly (558 vs 560 g/d, 2.26 vs 2.26 feed/gain) and there was no evidence of a sex X treatment interaction. From the results of this experiment, we conclude that thiopeptin is an effective growth promoting agent for growing swine.