In seronegative autologous bone marrow transplanted (ABMT) patients, a sustained cell-mediated immunity (CMI) has been shown to impair the antibody response after measles vaccination. To investigate if this might be caused by a preferential Th1 cytokine response, interferon (IFN)-gamma and interleukin (IL)-10 production of peripheral blood mononuclear cells (PBMC) was analyzed after measles antigen (M-ag) stimulation in vitro. The non-specific immune response was measured by IFN-alpha, and IL-12 analyses. Fifty non-vaccinated patients following ABMT or allogeneic bone marrow transplantation (BMT) were included. IFN-gamma production was significantly higher in patients with a retained CMI to measles than in patients without (2.3 vs 0.8 IU/ml; P = 0.01). Only a non-significant tendency was seen in IL-10 production (48.6 vs 26.7 pg/ml; NS), whereas no difference was found in IFN-alpha or IL-12 production. A positive correlation between IFN-gamma and IL-10 production was found (r(s) = 0.49; P < 0.001). After vaccination of 14 ABMT children, there was an increase in PBMC IFN-gamma production in vitro (2.5 vs <0.1 IU/ml; P < 0.05), whereas no changes were seen in the IL-10, IFN-alpha, or antibody levels. These results suggest that both Th1 and Th2 cytokine production are increased by M-ag stimulation in patients with a retained CMI to measles, but the Th1 response seems to be stronger. The preferential Th1 stimulation and increase in IFN-gamma production after vaccination may lead to a reduction in the humoral immune response which may explain the negative correlation between antibody production and T cell reactivity prior to vaccination.