Mycoplasma fermentans (strain 'incognitus') was incubated with HeLa cells for up to 96 h. After 24 h, mycoplasma organisms were demonstrated intracellularly by immunocytochemistry using mule anti-M. fermentans antiserum and gold labelling on ultrathin sections of both Lowicryl K4M and Araldite-embedded HeLa cells, the latter being treated with hydrogen peroxide. The Araldite-embedded cells were fixed with glutaraldehyde and osmium tetroxide in the presence of ruthenium red to stain the mucopolysaccharide surface components of both the procaryotic and eucaryotic cells. Intracellular localization of some M. fermentans organisms was confirmed by exclusion of ruthenium red from their membranes. Various numbers of mycoplasma organisms were seen per cell and occasionally some were within vacuoles, the membranes of which were also unstained by ruthenium red. The PG18 strain of M. fermentans and a strain of M. hominis were also detected intracellularly using similar methodology and homologous mule or rabbit antisera. The occasional presence of both apparently normal and some denser degenerate mycoplasmas in the same cell may indicate gradual degradation by phagolysosomal digestion.