We studied 10 intrauterine contraceptive devices by scanning and transmission electron microscopy. All the devices had material adherent to them. The amount of this material varied considerably. Many different morphologic types of bacteria were observed adherent to the devices, often buried in a thick biofilm. Occasionally a microcolony containing only a single bacterial morphotype was observed. Electron microprobe analysis revealed the presence of calcium in the biofilm formed by bacterial colonization. Very few types of bacteria were recovered by our cultural technique, suggesting that material must be scraped from the surface of the device, homogenized, and then cultured by means of selective media. An understanding of the mechanisms of adherence of bacteria to these devices could lead to the development of a device which will resist bacterial colonization.