1. Gliding motility ofMyxococcus xanthus FBt is reversibly inhibited by treatment with proteolytic enzymes and with 10-3 M ethylenediaminetetraacetic acid and by osmotic shock. Chloramphenicol inhibits regeneration of the ability to glide. Shearing and ultrasound treatments do not affect movement. 2. At least 18 periplasmic proteins are released by osmotic shock, as determined by SDS-polyacrylamide gel electrophoresis. These proteins are indistinguishable from those of the non-motile strain NM. Pronase treatment of FBt degrades most of the higher molecular weight periplasmic proteins. 3. Antisera prepared against periplasmic antigens of FBt and NM do not demonstrate strain-specific antigenic differences by immunodiffusion. NM does not agglutinate as well as FBt with either antiserum. Osmotic shocked FBt cells do not agglutinate. Both strains FBt and NM demonstrate a predominance of pole-to-pole agglutination. 4. These antisera inhibit gliding motility. Serum against FBt periplasmic antigens, adsorbed with FBt, does not inhibit gliding; adsorption with NM or osmotically shocked FBt only partially inhibits gliding. 5. Surface property differences among strains FBt, NM and SM (semi-motile) are indicated.