Cytokine-encoding DNA plasmids can act as 'genetic adjuvants', improving the immune response stimulated by co-administered DNA vaccines. We examined whether plasmids encoding the Th1 cytokine IFN gamma (pIFN gamma) or the Th2 cytokine IL-4 (pIL-4) have long-term effects on immune homeostasis when administered to adult mice, or alter immune maturation in neonates. Both plasmids boosted immunity against a co-administered vaccine, with pIFN gamma promoting the development of a Th1 response (characterized by the production of IgG2a antibodies), and pIL-4 preferentially stimulating a Th2 response (characterized by increased IgG1 antibody production). Both pIFN gamma and pIL-4 influenced the ratio of cells actively secreting Th1 versus Th2 cytokines, consistent with an effect on Th cell maturation. Interestingly, this effect persisted for only a few weeks and was not magnified by repeated plasmid administration. Cytokine-encoding plasmids had no long-term effect on the immune response of newborn or adult mice to subsequent antigenic stimulation, nor did they selectively induce the production of pathogenic anti-DNA autoantibodies. These results suggest cytokine-encoding plasmids may be safe as immune adjuvants.