Female DBA/2 mice have been shown to be relatively resistant to infection with Leishmania mexicana when compared with male mice. In order to determine the immunological basis behind this difference the draining lymph nodes from male and female DBA/2 mice were excised and the RNA extracted at different time-points following infection. Following reverse transcription, the polymerase chain reaction (PCR) was used to identify mRNA transcripts for interferon-gamma (IFN-gamma), tumour necrosis factor-alpha (TNF-alpha), interleukin-4 (IL-4), IL-10 and IL-12. The evolution of cytokine mRNA production was slow in both male and female mice as no newly synthesized transcripts were identified 5 weeks after infection. IL-10 was expressed constitutively in non-infected mice and was present throughout the experiment in all animals. By week 8, a clear dichotomy in cytokine mRNA expression was emerging between the resistant female and susceptible male mice. Whereas all females expressed IFN-gamma and one also expressed TNF-alpha only two out of five males expressed IFN-gamma and four out of five expressed TNF-alpha. The greatest lesion sizes at this time were recorded from those mice expressing TNF-alpha but not IFN-gamma. No differences in IL-4 or IL-12 were noted with transcripts for both cytokines present in both sexes at week 8. By week 12 males had developed large non-healing nodules and in females lesions had either disappeared or were slow growing. At this time only transcripts for TNF-alpha were present in males and only those for IFN-gamma were detected in females. Treatment of female mice following infection with IFN-gamma neutralizing antibody resulted in lesion growth equivalent to male mice. IFN-gamma production would, therefore, appear sufficient to limit the growth of L. mexicana in female DBA/2 mice while TNF-alpha production in the absence of IFN-gamma confers no protection to DBA/2 male mice.