Data concerning interleukin 10 (IL-10), a cytokine of Th2 lymphocytes, and its inhibition of Th1 lymphocytes from secreting interleukin 2 (IL-2) and interferon (IFN) are presented. It has been indicated that IL-10 also inhibits other cells from producing interleukin 12 (IL-12) and nitric oxide (NO). It is known that all these factors take part in the cell-mediated immune response and immunity. This inhibition may facilitate the multiplication of Treponema pallidum and the development of disease despite the presence of immunologically competent cells. It has also been demonstrated that in late latent syphilis, when Th1 lymphocytes are not able to produce IL-2 and IFN, the cells are able to produce only IL-12 and NO. This fact seems to suggest that these factors take over the immune function when cells are stimulated again by treponemes which, after many years of latency, begin to multiply. Thus, a high level of IL-12 and NO seems to be an indicator of the development of the third stage of disease.