Synd1 content in sera of mice challenged with 30 LD50 of Sterne strain spores intraperitoneally. The serum samples were either dot-blotted in duplicates onto membrane (A) or were used to cover wells of the ELISA plate (B) for immunodetection of Synd1 with antibody 281-2. Graphs represent average fold increases over control. In (A), two mice were used at the day of challenge, three mice at each day 1 and 2, and two mice at day 3. In (B), seven mice were used at each of the days 0, 1 and 2, and three mice at day 3. A calibration concentration-response curve obtained by triplicate measurements of control serum spiked with recombinant Synd1 is shown in (C). The bars indicate 95% confidence intervals. *P < 0.05, **P < 0.01.