The clone 4G7 recognizes a 90-kDa CD19 antigen that is present on human B lymphocytes. The CD19 antigen is present on approximately 7 to 23% of human peripheral blood lymphocytes at all stages of B cell maturation but is lost on terminally differentiated plasma cells. CD19 does not react with resting or activated T lymphocytes, granulocytes, or monocytes.