The bacterium Proteus mirabilis exhibits an unusual form of motility on solid surfaces known as swarming. During swarming, cells undergo a morphological change from short rods to highly elongated and flagellated rods called swarm cells. Swarm cells are highly energetic and move away from the parental colony in groups or "rafts." After swarm cells have migrated a certain distance, movement ceases and swarm cells revert back to short forms. Following a brief resting period, swarm cells are again formed and the cycle is repeated;Microscopic studies of swarming showed that extracellular slime is always associated with migrating swarm cells. In this study, improved methods of observing slime associated with swarming cells are presented. The use of toluidine blue O to stain slime on impression mounts and a fixation method consisting of hot sulfuric acid and sodium borate provide new ways to visualize slime. Extracellular slime of P. mirabilis IM 47 was isolated by centrifugation. A biological assay was developed based on the ability of isolated slime to promote swarming of a nonswarming mutant of P. mirabilis. For the assay, polycarbonate filters were dipped in the test material (slime) and placed on an agar plate to dry. The nonswarming mutant was inoculated onto the filter and swarming was examined after incubation at 35 C for 24 h. Biological activity was also found in slime which was purified with protease and ion exchange chromatography. Chemical assays showed that purification produced increased quantities of anthrone-positive carbohydrates and uronic acids;The role of slime in swarming is presently unclear. Slime may promote swarming by providing a reduced surface tension and lubricity necessary for swarm cell migration. Many functional similarities appear to exist between the slime of swarming P. mirabilis and the slime of gliding bacteria.