Mice of the C57BL/6 strain were studied to investigate the effects of a 20% total body surface area full-thickness burn on class-specific immunoglobulin synthesis. Five days after injury single cell suspensions of spleen and mesenteric lymph node cells were made. The number of class-specific immunoglobulin-bearing cells were counted by flow cytometry using Texas-red-labelled antibody to class-specific mouse immunoglobulin. The remaining cells were cultured with lipopolysaccharide for 4 days. The supernatant was tested for class-specific immunoglobulin by enzymelinked immunosorbent assay (ELISA). There was no change in the number of classspecific IgG and IgA lymphocytes, but there was a significant increase in IgM lymphocytes. Synthesis of immunoglobulin by 105lymphocytes was significantly increased in IgG and IgA cells, but decreased in IgM cells. Similar results were shown in spleen and mesenteric lymph node cells. Because the IgM response was much greater than others in the number of cells and immunoglobulin synthesis, we conclude that the decrease in responsiveness of IgM may contribute significantly to increased host susceptibility to infection after burn.