Total levels of urine and serum immunoglobulin IgM, IgG and IgA, and the E. coli-specific bacterial immunoglobulin response were determined by enzyme-linked immunosorbent assay (ELISA) in a rat model of acute urinary tract infection. High levels of urinary IgM were detected as early as day 3 post infection and then decreased to statistically insignificant levels. Peak levels of IgG occurred in the serum and urine on day 14. Urine and serum IgA levels remained low throughout the study period. The results demonstrate that in the rat model of acute urinary tract infection, IgM appears first in the urine and serum, and rapidly decreases. IgG then appears in the serum and urine followed by a late E. coli-specific immunoglobulin serum and urine response. Also, a non-specific component of the immunoglobulin response was noted in both the serum and urine. In the rat, IgA appears to play little or no role in the urine or in the serum response to the infection.