Mycoplasma incognitus is a recently described organism that was originally isolated by Shyh-Ching Lo from spleen and Kaposi's sarcoma (KS) tissue of patients with AIDS. The mycoplasm has since been found in selected AIDS autopsy cases in which there were tissues with unexplained areas of necrosis. Mycoplasma incognitus is now known to be closely related to Mycoplasma fermentans. In order to determine the incidence and distribution of M. fermentans/M. incognitus in an unselected series, we performed a retrospective immunohistochemical study of 42 HIV-positive autopsies from the Chicago AIDS autopsy series. One case (2.4%) was found to be positive. The patient was a 27-year-old white male homosexual who had no evidence of Kaposi's sarcoma nor of any other neoplastic disorder and who died of Pneumocystis carinii pneumonia. Antigen was present in the macrophages of the patient's spleen, liver, and bone marrow but was not associated with areas of necrosis. All other tissues sampled from this case (brain, heart, testis, lymph nodes) were negative. In the remaining autopsies, organs of the reticuloendothelial system as well as other randomly selected tissues (e.g., lung, heart, testis, pancreas) were screened and found to be negative. Tissues from several non-AIDS autopsies were also found to be nonreactive for M. fermentans/M. incognitus. We conclude that M. fermentans/M. incognitus is not a common agent found in the Chicago AIDS population; that it may be identified in tissue without morphologic abnormality; and that it is not randomly present in non-AIDS autopsies.(ABSTRACT TRUNCATED AT 250 WORDS)