We established a human burn isolate of Proteus mirabilis as an experimental pathogen. Infliction of a nonfatal scald injury (30%) rendered rats highly susceptible to lethal surface infection with this isolate. Dose-response experiments indicated that the lethal inoculation dose (50%) was less than 10(3) organisms per square centimeter. Histopathologically, surface colonization was followed by progressive growth with subsequent invasion of viable tissue. The invasion was not characterized by the perivascular or perineural lesions observed in experimental Pseudomonas burn sepsis. Bacteriologic examinations showed moribund animals to be bacteremic with the test strain and to have wound biopsy counts exceeding 10(6) organisms per gram of tissue. The role of bacterial motility as a virulence factor in this surface infection was investigated. Substrains selected for loss of subsurface spreading in soft agar lost virulence. This model of burn infection with a member of the Enterobacteriaceae should be used to evaluate topical and parenteral antimicrobial agents needed for the control of wound infections caused by such agents.