Statistical procedures are presented for determining ordinal and disordinal aptitude-treatment interactions with linear and curvilinear data. The paper presents a method for testing the homogeneity of group regressions for a single aptitude and provides models for expanding this test to linear and curvilinear regression planes. Procedures are presented for constructing appropriate tests of significance and for isolating the specific source of interaction in complex aptitude-treatment interactions for which there are multiple aptitudes. (Author)