M. hominis is commonly found as part of the normal flora in the female genital tract, but several studies have shown that it may be involved in a variety of urogenital infections. The basis for clinical manifestations in some patients has varyingly been attributed to host and M. hominis factors. The host factors involved in the infection process are largely unknown. M. hominis have no cell wall and outer membranes, and at present it seems plausible that M. hominis possesses genetic systems allowing the bacteria in vivo to alter its antigenic structure on the membrane surface and consequently circumvent the host immune system. The studies of M. hominis have shown that the antigenic variation is pronounced between surface exposed membrane proteins from different isolates. The genetic background for this variation has been investigated for three surface exposed membrane proteins: P120, Lmp, and Vaa. P120 and P120' are similar proteins in M. hominis without any homology to other known proteins. A hypervariable region in the otherwise conserved P120 protein seems to be very antigenic in patients with immunologically verified M. hominis infection. The remaining part of P120 as well as the entire P120' protein do not seem to elicit significant antibody formation. Two genes in M. hominis, lmp1 and lmp3, contain numerous highly similar 0.5 kb tandem repeats at their 3'-end. The proteins, Lmp1 and Lmp3, are synthesized from the lmp1 and lmp3 genes, respectively. Lmp1 shows size variation among M. hominis isolates. M. hominis isolates investigated in detail show that the size variation of Lmp1 corresponds to the variation in number of 0.5 kb repeats contained within the lmp1 gene. Lmp3 appears to have a lesser tendency to size variation. M. hominis isolates were found with deletions involving the lmp1 stop codon leading to translation of the downstream gene lmp2 and expression of a chimeric Lmp1-Lmp2 protein. The number of repeated elements in the lmp1 gene of a M. hominis isolate correlates with the extent of anti-Lmp antibody induced agglutination between the bacteria. Vaa is a protein involved in cell adherence. vaa is a single copy gene containing tandem repeated elements like the lmp gene family. The number of repeats in the Vaa protein differs between M. hominis isolates leading to size variation. It has been suggested that the number of repeated elements is of importance in the bacteria-host adhesion process. Beside the size variation Vaa demonstrates phase variation due to frequent frame shift mutation in a specific region near the 5'-end of the structural gene. Based on the investigations of M. hominis and other mycoplasmas several genetic mechanisms seem to be responsible for the antigenic variation of surface exposed membrane proteins in mycoplasmas: 1) variation in protein size due to insertions or deletion of repeated elements in the structural gene, 2) presence of multi-gene families, and 3) phase variation due to mutations in the promotor region or the coding region. The influence of specific antibodies on antigenic variation of membrane proteins has not been studied in greater detail in mycoplasmas. In M. hominis it was investigated whether the presence in the culture medium of monoclonal antibodies directed against the repeated elements in the M. hominis Lmp proteins would affect gene structure and consequently protein expression. The presence of anti-Lmp antibodies resulted in overgrowth of bacteria with specific deletions in the repeated elements of lmp1 leaving the lmp3 gene unchanged. The precise mechanism leading to the dominance of M. hominis isolates with fewer 0. (ABSTRACT TRUNCATED)