An enzyme-linked immunosorbent sandwich assay (ELISA) has been developed to detect and quantify both the K88 ®mbrial antigen and the concentration of bacterial cells in fecal and other samples using anti-®mbrial chicken egg-yolk and anti-®mbrial rabbit serum antibodies. The assay has a sensitivity of 40 ng ml À1 for K88 ®mbrial antigen and 10 7 CFU ml À1 for intact cells and cell homogenate. The inter-and intra-assay coef®cients of variation (CV) for intact cells, homogenates, and ®mbrial antigens were similar, suggesting that reproducible results can be obtained with any of the three preparations. In all cases the CV increased with decreasing concentration of the antigen, especially when very low concentrations of antigen were used. Fimbrial antigens had lower CV (values ranged from 2 to 37%, depending on its concentration) than standards prepared from either whole cells or homogenized cells (values ranged from 1 to 67%). The correlation between the actual number of cells as counted and the number as estimated from the ELISA using the amount of ®mbrial antigen was r = 0.98, P`0.001. The anti-K88 polyclonal antibodies from chicken serum and egg yolk had little or no cross-reactivity with K99, 987P ETEC. The correlation (r) between the fecal score for diarrhea and the number of E coli in the fecal swab was high, r =0.80 (n = 64, P`0.0001). The assay is sensitive and speci®c and provides a good estimate of the amount of ®mbrial protein or the number of K88 ETEC in the sample.