A national random sample of over 14,000 high school seniors was studied with respect to socio-cultural differences in responses to cognitive test items. Six different cognitive tests and ten different groups were analyzed. The tests were: vocabulary, picture-number, reading, letter-groups, mathematics, and mosaic comparisons. The groups were: American Indians, Blacks, Mexican-Americans, Puerto Ricans, Other Latin-Americans, Oriental-Americans, White Northeastern, White North Central, White Southern, and White Western. Proportions of each group responding correctly to each item of each test were computed and then transformed to an interval scale of delta-values. The delta-values for the White North Central group were then cross-plotted with each of the other groups to yield an elliptical pattern of points for each comparison. The major axis of the ellipse for each cross-plot was determined and the distance of each item-point from it computed. These distances were used to create a vector index of cross-cultural stability. Items having notable patterns of instability were examined closely for factors which might explain such an outcome. The greatest instabilities were noted among the vocabulary items. These vocabulary instabilities appeared to be attributable to linguistic differences, primarily those existing between Spanish-speaking groups and other groups. Vocabulary items involving cognates were relatively easier for the Spanish-speaking groups. It was also observed that reading test items having material relevant to black culture was relatively easier for blacks than were other items in the test battery. (Author/MV)