It is now firmly established that the mollicutes are true eubacteria. They have evolved regressively (i.e., by genome reduction) from gram-positive bacterial ancestors with a low content of guanine plus cytosine in DNA--more specifically, from certain clostridia. Many of their properties, such as small genome size, small number of rRNA operons and tRNA genes, lack of a cell wall, fastidious growth, and limited metabolic activities, are seen as the result of this evolution. Other properties, such as the anaerobiosis of their earliest evolving members (anaeroplasmas and asteroleplasmas), the high adenine-plus-thymine content of their DNA, their lack of sensitivity to rifampin, and the regulatory signals for the transcription of their DNA, have been inherited from their eubacterial ancestors. However, the mollicutes are not simply wall-less gram-positive bacteria. They have properties of their own. High adenine-thymine pressure has resulted in a particular codon usage, where, for instance, UGA is read as tryptophan and not as stop. These organisms occupy unique ecological niches and have developed peculiar systems for pathogenicity, cell adhesion, antigenic variation, and (in the case of the spiroplasmas) helical morphology and motility. The putative role of certain mollicutes as cofactors in the development of AIDS may involve their mitogenicity, their superantigenicity, and their ability to induce cytokines.