In an effort to appraise the degree to which response consistencies on the MMPI are attributable in widely differing populations to the general stylistic dimensions of acquiescence and desirability on the one hand, and to particular content consistencies on the other, two studies on two diverse samples were performed. By applying factor analysis to correlations among MMPI clinical and validity scales scored separately for "true" and "false" items, along with five specially constructed reference scales to measure tendencies to endorse items at varying levels of desirability, relatively precise estimates of the proportion of variance attributable to response style and to content were obtained. The two samples employed--hospitalized neuropsychiatric patients and college students--yielded results consistent with those obtained from a previous study of prison inmates, in that the dominant role of acquiescence and desirability response styles was emphasized. In all three samples two very large factors appeared, identifiable as acquiescence and desirability. The massive response style effects on the MMPI, while contributing to convergent validity in grossly separating normals from mental patients, are interpreted as reducing the possibilities for discriminant validity, as in the differential diagnosis of psychopathological syndromes. Where the latter is important, experimental controls aimed at sharply reducing response biases are recommended.