Protective immune responses to intracellular pathogens such as Brucella abortus are characteristically Th1-like. Recently we demonstrated that heat-killed B. abortus (HKBa), a strong Th1 stimulus, conjugated to ovalbumin (HKBA-OVA), but not B. abortus alone, can alter the antigen-specific cytokine profile from Th2- to Th1-like. In this report we study the ability of a single injection of B. abortus to switch a Th2 to a Th1 response in immature mice. One-day- and 1-week-old mice were given a single injection of B. abortus in the absence or presence of OVA, and at maturity mice were challenged with an allergenic preparation, OVA with alum (OVA-A). B. abortus given without OVA did not diminish the subsequent Th2 response in either age group. In contrast, mice receiving a single injection of B. abortus-OVA at the age of 1 week, but not those injected at the age of 1 day, had reversal of the ratio of OVA-specific Th1 to Th2 cells and decreased immunoglobulin E levels after allergen challenge as adults. Within 6 h both 1-day- and 1-week-old mice expressed interleukin-12 p40 mRNA following either B. abortus or B. abortus-OVA administration. However, only the 1-week-old mice exhibited increased expression of gamma interferon (IFN-) mRNA. The absence of the early IFN- response in 1-day-old mice may explain their inability to generate a Th1 memory response. These results suggest that at early stages of immune development, responses to intracellular bacteria may be Th2- rather than Th1-like. Furthermore, they suggest that the first encounter with antigen evokes either a Th1- or a Th2-like response which becomes imprinted, so that subsequent memory responses conform to the original Th bias. This has implications for protection against infectious agents and development of allergic responses.