Polyclonal B cell activation is a well-described feature of systemic lupus erythematosus (SLE), but the immune mechanisms leading to this activation are unclear. To gain insight into these processes, we extensively characterized the activated peripheral blood B cell populations in SLE. PBMC from lupus patients and healthy controls were stained with various combinations of conjugated Ab to identify distinct peripheral B cell subsets, and activation was assessed by measurement of forward scatter and CD80 or CD86 expression using flow cytometry. SLE patients had altered proportions of several B cell subsets, many of which demonstrated increased activation as assessed by forward scatter. This activation occurred at an early developmental stage, as B cells in the transitional (T2) stage were already significantly larger than those seen in controls. Increased proportions of CD80-or CD86-expressing cells were also seen in multiple B cell subsets, with the most striking differences observed in the naive CD27− CD23+ population. Within the CD23+ subset, increased costimulatory molecule expression was most pronounced in an IgD+ IgM low population, suggesting that activation follows Ag engagement. Although controls also had IgD+ IgM low CD23+ cells, they were reduced in number and not activated. Thus, there is an altered response to Ig receptor engagement with self-Ags in lupus.