The phenomenon of simultaneous differential item functioning (DIF) amplification and cancellation and the role of the SIBTEST computer program in detecting it were studied. A variety of simulated test data was generated for this purpose. In addition, the following real test data were used: (1) American College Testing program data for 2,115 males and 2,885 females in mathematics; (2) National Assessment of Educational Progress (NAEP) history test data for 1,225 males and 1,215 females; and (3) NAEP data for 1,711 whites and 447 blacks. The results from both simulated and real data, as the theory of R. Shealy and W. F. Stout suggests, show that SIBTEST is effective in detecting DIF amplification and cancellation (partially or fully) at the test score level. Finally, methodological and substantive implications of DIF amplification and cancellation are discussed. Ten tables present analysis results. (SLD)