Group G streptococci which have been isolated from the oral flora of rats are also normal inhabitants of the human skin, oropharynx, gastrointestinal tract, and female genital tract. This group of streptococci can cause a wide variety of clinical diseases in humans, including septicemia, pharyngitis, endocarditis, pneumonia, and meningitis. Ten days after oral gavage with 7,12-dimethylbenz[a]anthracene, 12 of 22 two-month-old, female, outbred, viral-antibody-free rats presented with red ocular and nasal discharges and marked swelling of the cervical region. Various degrees of firm, nonpitting edema in the region of the cervical lymph nodes and salivary glands as well as pale mucous membranes and dehydration were observed. Pure cultures of beta-hemolytic streptococci were obtained from the cervical lymph nodes of three rats that were necropsied. A rapid latex test system identified the isolates to have group G-specific antigen. These streptococcal isolates fermented trehalose and lactose but not sorbitol and inulin and did not hydrolize sodium hippurate or bile esculin. A Voges-Proskauer test was negative for all six isolates. Serologic tests to detect the presence of immunoglobulin G antibody to rat viral pathogens and Mycoplasma pulmonis were negative. Histopathologic changes included acute necrotizing inflammation of the cervical lymph nodes with multiple large colonies of coccoid bacteria at the perimeter of the necrotiz zone. To our knowledge, this is the first report of naturally occurring disease attributed to group G streptococci in rats.