We reported that preadsorption of patient serum with heat killed E. coli increased the specificity of ELISA for antibodies to Borrelia burgdorferi. That procedure required extra specimen handling and a preincubation. We report the use of a soluble E. coli antigen fraction that is included in serum diluent, eliminating additional steps. Sera from 220 individuals were tested for antibodies to B. burgdorferi. Twenty sera were obtained from patients with Lyme disease and 200 sera were from a population that included healthy controls and patients with different inflammatory conditions (viral infections and various rheumatic disorders). Testing was performed using either a standard serum diluent or one containing soluble E. coli antigen fraction. Results demonstrate that inclusion of soluble E. coli antigen fraction in serum diluent increased assay specificity from 88% for the standard protocol to 98%, with no change in test sensitivity.