Natural killer (NK) cells are important in immune function and appear, in part, to be regulated by the CNS. The authors compared NK cell activity and MMPI scores of 111 healthy college students and found weak but statistically significant correlations between NK values and psychopathology for 10 of 12 scales. Students with the highest NK values had a "healthier" MMPI profile than those with the lowest. Students with high MMPI scores (T greater than 70) had NK values below the sample median. These findings support theories of interaction between mental state and immune status, but the mechanisms and direction of interaction remain largely unexplored.