g Clostridium difficile is an anaerobic, Gram-positive, and spore-forming bacterium that is the leading worldwide infective cause of hospital-acquired and antibiotic-associated diarrhea. Several studies have reported associations between humoral immunity and the clinical course of C. difficile infection (CDI). Host humoral immune responses are determined using conventional enzyme-linked immunosorbent assay (ELISA) techniques. Herein, we report the first use of a novel protein microarray assay to determine systemic IgG antibody responses against a panel of highly purified C. difficile-specific antigens, including native toxins A and B (TcdA and TcdB, respectively), recombinant fragments of toxins A and B (TxA4 and TxB4, respectively), ribotypespecific surface layer proteins (SLPs; 001, 002, 027), and control proteins (tetanus toxoid and Candida albicans). Microarrays were probed with sera from a total of 327 individuals with CDI, cystic fibrosis without diarrhea, and healthy controls. For all antigens, precision profiles demonstrated <10% coefficient of variation (CV). Significant correlation was observed between microarray and ELISA in the quantification of antitoxin A and antitoxin B IgG. These results indicate that microarray is a suitable assay for defining humoral immune responses to C. difficile protein antigens and may have potential advantages in throughput, convenience, and cost. C lostridium difficile is the leading worldwide infective cause of hospital-acquired and antibiotic-associated diarrhea, imposing a considerable financial burden on health service providers in both Europe and the United States (1-3). Infection causes a spectrum of clinical presentations ranging from an asymptomatic carrier state to severe fulminant colitis and death (4). Following successful treatment, an estimated 20% to 30% of patients with primary C. difficile infection (CDI) develop recurrence of symptoms, caused by either relapse of the original infection or reinfection with a new strain (5).This anaerobic and spore-forming bacterium exerts its major pathological effects through two proinflammatory and cytotoxic protein exotoxins, TcdA (toxin A) and TcdB (toxin B) (6). Nontoxin virulence factors, such as surface layer proteins (SLPs) and cell wall proteins, have also been described and may play a role in disease expression (7-9).The majority of healthy adults have detectable antibodies to C. difficile TcdA and TcdB in their sera that are thought to arise from colonization in infancy or from repeated exposure to C. difficile in adulthood from the environment (10, 11). Several clinical studies suggest that adaptive humoral immune responses, in particular to TcdA and TcdB, may influence clinical outcomes of CDI (12). Most notably, a landmark study in 2000 reported that a low IgG titer to TcdA, but not TcdB, at the time of infection is associated with development of symptomatic disease (13). More recently, the same group demonstrated an association between median IgG titers to TcdA and 30-day all-cause mortality (14). Several r...