Lipid-associated membrane proteins (LAMPs) from 14 Mycoplasma hominis isolates or strains share similar protein and antigenicity profiles. Of 31 human immunodeficiency virus-infected patients from whose samples M. hominis was cultured, 28 tested strongly positive for serum antibodies to M. hominis LAMPs. The remaining 3 serum samples showed low antibody titer to LAMPs from all of the 14 M. hominis isolates or strains, which was likely the result of the compromised immune systems of the patients. Thus, M. hominis LAMPs as a whole are homogenous in antigenicity within the species, despite having many different serotypes. Serological study involving 564 healthy blood donors and 211 patients attending sexually transmitted disease clinics by LAMPs showed that general populations were widely exposed to M. hominis. Women were infected with M. hominis at a younger age than were men. The prevalence of infection increased markedly among sexually active persons.