1. A microscopy system giving good fluorescence at 400 times magnification or greater is essential, and in effect this means a vertical illumination incident light system of the Ploem type, with optical quality lenses and objectives. 2. Enumeration of positive cells must allow for the probability of contamination of separated mononuclear cell preparations by monocytic cells. 3. The specificity of the immunofluorescent reagents used must be rigorously checked, and the presence of aggregated labelled IgG excluded. 4. The concurrent use of an independent method of identifying T-cells provides an important check, especially when there is a possibility of anti-lymphocytic antibodies being present. 5. In chronic lymphocytic leukemia, tests for surface markers for T and for B cells may permit detection of the less common T-cell leukaemia, which may have a graver prognosis. 6. Immunofluorescence tests a morphological lymphocytic marker. In the more subtle forms of lymphocytic disturbance, there is a need for tests of lymphocyte function.