Over 14,000 high school seniors were studied with respect to sociocultural differences on cognitive test item responses. 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 equal interval scales 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 the instabilities. It was concluded that certain vocabulary items were unstable for some Spanish-speaking groups. These vocabulary instabilities were attributed to coincidental cognate influences operating to make some English vocabulary items relatively easier for the Spanish-speaking groups. It was also observed that reading test items containing material relevant to black culture were relatively easier for blacks than were other items in the test battery. (Author/RC)