Demonstration of surface Ig on lympohcytes is subject to interference by the formation of immune complexes between the fluorochrome tagged antibodies and minute amounts of residual serum Ig present in the fluid phase or loosely bound to the Fc receptor. These newly formed immune complexes accordingly bind to Fc receptors, including those of the Fc-positive Ig-negative third population, and result in falsely elevated percentages of Ig-bearing cells. In principle, this immune complex formation can occur with any soluble antigens, but in practice it is most commonly encountered with IgG and to a lesser extent with IgM or IgA determinations. The use of fluorochrome tagged F(ab')2 fragments of antibodies in fluorescence results in an immune complex without an exposed Fc region that does not bind to Fc receptors. Methods for preparation of the F(ab')2 fragments are described. The use of latex ingestion as an aid for the identification of monocytes has the additional effect of allowing autoreactive anti-lymphocyte antibodies to elute from the cell surface and thereby diminishes this additional source of false positive surface fluorescence.