A microtitre method was developed to screen Escherichia coli from 48 patients with ulcerative colitis and 25 controls for serum resistance. Bactericidal resistance was indicated by a change in colour of indicator due to acid production by viable organisms and quantitated by a change in absorbance. The method clearly differentiated between organisms confirmed as resistant or sensitive by conventional techniques. Twenty four (50%) disease and 14 (56%) control E coli specimens showed serum resistance. Bactericidal competence of sera from patients with ulcerative colitis was assessed by incubating sensitive E coli with sera from 10 patients with ulcerative colitis and pooled normal serum. All sera effectively reduced viable counts to less than 6% of original inoculum. This study shows that serum samples from patients with ulcerative colitis are bactericidally competent and that there is no increase in the number of serum resistant E coli in patients with ulcerative colitis.