The colony-stimulating factors (CSFs) are cytokines involved in the production, differentiation, and activation of host phagocytes. During murine infection with Chlamydia trachomatis (MoPn), plasma CSF levels increased in euthymic (nu/+) and athymic (nu/nu) BALB/c mice. Levels declined later in infection, with the nu/+ mice resolving the infection but the nu/nu mice succumbing by day 16. Either live or heat-killed Chlamydia organisms could induce CSF increases on day 7 postchallenge in nu/+ mice; however, by day 14, only mice challenged with live organisms maintained high plasma levels. CSFs were also produced by spleen cells of nu/+ and nu/nu mice in response to Chlamydia antigen. Spleen cell CSF production was detectable by days 3 to 5 postinfection. In nu/+ mice, spleen cell CSF production was elevated throughout the rest of the time course but in nu/nu mice fell significantly at day 14. Like the plasma CSF activity (CSA) production, spleen cell CSA production on day 7 was seen in mice challenged with either live or heat-killed Chlamydia organisms, but on day 14 only nu/+ mice challenged with live organisms maintained significant CSA production. To further characterize the T-cell dependence of CSA production, spleen cells of nu/+ mice were depleted of T cells or T-cell subsets before producing supernatants. On day 14 postinfection, the CD4+ lymphocyte was the major producer of CSFs. Additionally, there were different types of CSFs secreted by nu/+ and nu/nu mice as determined by the ability of spleen cell supernatants to support the granulocyte-macrophage CSF/interleukin 3-dependent cell line FDCP-1. Supernatants from nu/+ mice had 4 to 8 times the level of FDCP-1 CSF activity of the supernatants from nu/nu mice. These results support the evidence that nu/+ mice were producing some CSFs by T-cell-dependent mechanisms. This is the first report of CSF production in vivo during Chlamydia infection. Furthermore, we show that CSFs are produced by both T-cell-dependent and T-cell-independent mechanisms. The capacity of the CSFs to increase the production and effector function of phagocytes may be important to host defenses.