Monoclonal antibody 38.13 was raised against the Burkitt lymphoma cell line Daudi and has been previously shown to recognize a globotriaosylceramide (pk) determinant expressed selectively on Burkitt and a few other B-cell lymphomas. We report here that 38.13 binds to a subset of normal B lymphocytes in germinal centres of secondary lymphoid follicles as well as to blood vessels. In malignant populations equivalent to various developmental compartments of the B-cell lineage, only those with a follicular phenotype express the pk determinant at detectable levels. The expression of this tumour-associated antigen, in common with many others, is therefore regulated in concert with cell lineage and maturation status.