Lymphocyte shape changes are detectable as changes in the forward light scatter (FLS) profile of a Becton-Dickinson FACStar flow cytometer, with polar cells giving lower values of FLS. The FACScan cell analyser does not detect these changes. Freshly isolated lymph node lymphocytes are round, but when stimulated, for example by incubation with cultured high endothelial cells (HEC), a proportion change shape and become polar. The FLS profiles of lymphocytes stained for T and B cell surface markers show that both subsets change shape in response to HEC, but in a consistently different manner. Light microscopy of sorted T and B populations indicates that equal proportions of both cell types change shape, but that T cells are more likely to become highly elongated.