Mononuclear and polymorphonuclear cells isolated by Ficoll-hypaque density-gradient sedimentation from peripheral blood of 25 burn patients at 3-to 4-day intervals during hospitalization were subjected to morphologic analysis. The aim was to identify all cells isolated and to examine the cell population characterized as B-lymphocytes. Patterns of response comprised of lymphocytes, monocytes, neutrophils, and precursor cells in varying stages of maturity were delineated. Shortly after thermal trauma, isolated cell pools from all patients contained high proportions of immature cells. In surviving patients during convalescence, the cell pools had increasing proportions of mature cells and corresponding declines in immature cell types. Patients who died did not produce large populations of mature cells at any time during their hospitalization. Cells identified morphologically as lymphocytes were accounted for by E-rosette assay (T-lymphocytes). Cells previously identified as B-lymphocytes appeared morphologically to be monocytes, were positive for Fc and complement receptor activity, and were phagocytically active.