The density of HLA determinants on the cell surface was studied in relation to HLA capping in childhood acute lymphoblastic leukemia (ALL). Analysis was performed with flow cytometry (FACS) and a subjective labeling score. These two methods gave comparable results. In T-ALL high percentages of capped cells were related to low numbers of HLA determinants. In c-ALL we found the opposite. This study shows that capping capacity and number of HLA determinants in childhood ALL are inversely related.