Abnormalities of the immune response are commonly observed after surgery. In many cases, they are part of a physiologic rather than of a pathologic response to trauma. In this study we show that after elective surgery in otherwise healthy subjects the B cell compartment is deeply affected, as documented by the appearance, 7 days after the intervention, of circulating lymphoblastoid B cells spontaneously secreting in vitro IgG and IgA antibodies. Analogous lymphoblastoid B cells have been described after in vivo immunization and represent a sensitive marker of the B cell response against the immunizing antigen. To better understand the origin of the reaction, we have analyzed the specificity of the antibodies secreted in culture supernatants. We show that the antibody response is polyclonal, since low titers of antibodies against several different bacterial antigens--such as tetanus toxoid, pneumococcal capsular polysaccharides (PCPs), and the lipopolysaccharides (LPSs) of several enteropathogenic strains of Escherichia coli--are detected. This response seems to reflect the previous immunologic experience of the single patient and to be caused by antigens released from traumatized tissues or absorbed through breaches in skin or mucous membranes.