〔Abstract〕 Objective To explore the causal relationship between 46 phenotypes(including 15 seropositive case-control phenotypes and 31 quantitative antibody-measurement phenotypes) and ulcerative colitis(UC) using two-sample bidirectional Mendelian randomization(TSMR). Methods Single nucleotide polymorphisms(SNPs) significantly associated with the relative abundance of the 46 antibody sera were extracted as instrumental variables according to preset thresholds. Summary statistics for UC were obtained from the OPEN GWAS database(n=47 745). MR-Egger regression, weighted median method(WME), inverse variance weighting(IVW), the simple mode method(SM), and weighted multitude method(WM) were used to estimate the causal relationship between antibody levels and UC, primarily using the IVW method. The results were assessed according to the effect indicator dominance ratios(OR) and 95% confidence intervals(CI). Sensitivity analysis, heterogeneity test, gene pleiotropy test, and outlier test(MR-PRESSO) were combined to verify the stability and reliability of the results, and the causal association study was performed again using reverse Mendelian randomization(MR). Results IVW results showed that Epstein-Barr(EB) virus EA-D antibody levels(OR=0.806, 95%CI=0.693-0.939, P