In studies on the antigenic structure of shigellae, an anodically-moving thermolabile antigen (ATA) was found, which furthermore could be detected in many other enterobacteriae (9, 10). ATA is a glycoprotein with a high molecular heterogeneity, resulting from aggregates of a subunit with a molecular weight of about 22000 Daltons. In the present paper the antigen was localized on the cell surface of several species by means of the immunoferritin technique. Antibodies against the purified ATA were raised in rabbits and were coupled with ferritin using glutaraldehyde. The antigen was found focally distributed over the whole circumference of the cell. According to the location of the ferritin granules, the ATA is tightly attached to the outer membrane. Especially some rough forms of the bacteria were heavily labelled on their surface. From the results obtained we conclude that in the smooth form the polysaccharide side chains of the somatic antigen cover the ATA.