In order to study the effect of in utero diethylstilbestrol (DES) exposure on the immune system of adult women, the blastogenic response of peripheral blood lymphocytes to two mitogens was compared in eight DES-exposed patients and in eight age-matched controls with normal menstrual cycles and proven fertility. As measured by the uptake of 3H-thymidine (mean [+/- standard error]), response to the T-cell mitogen phytohemagglutin (PHA) was significantly higher (P less than 0.002) in cells of DES-exposed women (88.6 +/- 5.7 X 10(3) cpm) than in controls (44.0 +/- 8.9 X 10(3) cpm) at the lowest dose of mitogen tested (0.125 microgram/ml). Moreover, lymphocytes of DES-exposed subjects showed maximal blastogenic response to PHA at a concentration (0.125 microgram/ml) two to four times lower (P less than 0.002) than controls (0.25 microgram/ml to 0.5 microgram/ml). Cells of both DES-exposed subjects and controls were maximally responsive to pokeweed mitogen (PWM) at the lowest dose tested (0.625 microgram/ml). These findings suggest that in utero DES exposure is associated with a hyper-reactive immune response during the reproductive years.