This article reports three studies that taken together support two hypotheses: (a) that the stressed power motivation syndrome is associated with relatively low natural killer cell activity (NKCA) and (b) that the unstressed affiliation motivation syndrome is associated with higher NKCA. In Study 1, college students who were relatively high in stressed power motivation had significantly lower NKCA than did their peers. In addition, students high in unstressed affiliation motivation had significantly greater NKCA than did those showing less evidence of this syndrome. Study 2 replicated these findings on a sample of middle-class men. In Study 3, which tested the hypotheses among adult patients from a Health Maintenance Organization, results were in the same direction but less significant. Meta-analyses clearly indicate that the combined evidence from the three studies reliably supports both hypotheses.