The production of interleukin-6 (IL-6), which is known as a B cell differentiation factor, by peritoneal macrophages from mice with a chronic lactic dehydrogenase virus (LDV) infection was compared with that from uninfected mice. The same amounts of IL-6 were detected in the culture supernatant of macrophages from LDV-infected mice as those from uninfected mice. Furthermore IL-6 production of macrophages from LDV-infected and uninfected mice was not affected by the addition of indomethacin. These results suggested that many immunological alterations seen in LDV-infected mice may not be due to, at least in part, altered IL-6 production ability of macrophages and the IL-6 production may not be affected by cyclooxygenase-derived products.