Immunofluorescence labelling of peripheral blood mononuclear cells is usually done on cells in suspension. This paper describes a procedure based on immuno-alkaline phosphatase staining of routine blood smears. The advantages of this method are that a few drops of blood are sufficient for labelling multiple lymphocyte subpopulations; smears may be stored for long periods before labelling; it is unnecessary to isolate a mononuclear cell fraction before labelling; labelled preparations can be stored; and the morphological features of labelled cells are shown clearly. The technique was used to label T cells and their subsets, B cells, and HLA-DR antigen in blood smears from 15 normal donors, from 7 patients with infectious mononucleosis, from 1 patient with clinically proven AIDS, and from 1 symptom-free subject at risk of AIDS. The normal T helper/suppressor ratio of 1·95 was reversed in all of the last three groups of subjects, the mean being 0·34 for infectious mononucleosis; the value was 0·22 in the AIDS patient. Immuno-alkaline phosphatase labelling of routine blood smears seems to be a valuable method for studying abnormalities in circulating lymphocyte subpopulations and lends itself to mass screening for altered T helper and T suppressor subjects-for example, in blood donors.