The internal structure of the Minnesota Multiphasic Personality Inventory (MMPI) was examined, with emphasis on the assessment of psychopathology. A factor analytic method was derived to provide precise estimates of the contributions of both content and consistent response style to total variance. Both content and response style determinants were evaluated and contrasted on the MMPI in three different populations. Both acquiescence and desirability were highlighted, and new measures of both styles were constructed. Samples included 201 male inmates of state correctional institutions; 119 male and 75 female hospital patients from a state neuropsychiatric hospital; and 160 male and 174 female undergraduate students. All groups showed clear individual differences in the consistent tendency to endorse desirable item content. Although the degree of undesirable item endorsement was higher for the patients than for the college students, remarkable stability was found in both samples. In analyses of all three groups, approximately three-fourths of the common variance and about one-half of the total variance was interpretable in terms of acquiescence and desirability response styles. Variance associated with the remaining small factors was attributable to item overlap and was of little significance. Important questions regarding the administration and interpretation of the MMPI for differential assessment of psychopathological conditions were raised.