It has generally been assumed that CLL B cells do not contain cytoplasmic immunoglobulin (Cylg), although these cells express surface membrane immunoglobulin (Smlg). The present study, in which Smlg and Cylg determinations were performed simultaneously using leukemia B cells from 20 patients with CLL, demonstrates that Cylg staining was detectable in each CLL cell population examined and that the intensity of cytoplasmic immunofluorescent staining in each instance was moderate to strong. Both Smlg and Cylg light chain determinations indicated monoclonality in all 20 cases. The heavy chain class of Smlg and Cylg in each CLL cell population was not uniformly comparable, however. The majority (15 cases) of CLL B cells contained mu heavy chain Cylg and mu and delta heavy chain Cylg was demonstrated in the remaining 6 cases. In contrast, the Smlg phenotypes appeared heterogeneous with both gamma and alpha chain determinants found associated with mu or mu and delta chains on the same leukemia cell populations. This apparent polyclonal Smlg staining pattern was most likely due to nonspecific adsorption of the patient's own serum Ig by Fc receptors on CLL B cells. It is concluded that the great majority of CLL B cells contain detectable Cylg and that Cylg determination is superior to Smlg phenotyping in documenting the monoclonality of CLL.