The definition of differential item performance (DIP), often referred to as item bias, is discussed. DIP is suggested as a comprehensive term to encompass item bias (item invalidity which is unfair to certain population subgroups) and instructional bias (a valid reflection of group differences in instruction or background). This study investigated the plausibility of an instructional bias interpretation of DIP as it results from gender differences on mathematics achievement items. The data from a national administration of the Mathematics Usage subtest of the ACT Assessment was used in the investigation. The results indicated that there was the large instructional effect as predicted. However, there was also a smaller, gender effect on the performance of some items. (Author/DWH)