A number of markers which have been proposed to identify B cell subsets have been reassessed on human B cells, using an immunofluorescence technique optimized for sensitivity and an analytical mode which yields histograms showing the distribution of fluorescence on B cells. The results show that CD38, CD22, CD23, FMC6, and anti-IgM react with all blood B cells, albeit with a broad and complex distribution of fluorescence. CD5, CD9, CD10, CD43, and IgD can be regarded as subset markers since they give clearly bimodal distributions of fluorescence intensity. CD5 staining showed at least three populations, with a small number (3-5 per cent) of cells brightly stained and a population of variable size staining weakly. No clearly defined populations were seen with CD45R0, although staining was slightly above background. An antibody against the LAM-1 molecule reacted with all blood B cells. Expression of the IL-2 receptor p55 chain (CD25) was clearly bimodal, whereas the p75 chain was essentially negative on B cells. The relationship between subsets in blood and subsets in tissue, and between subsets identified by different markers in blood, is discussed.