The labelling index (LI) of untreated B cell non-Hodgkin's lymphoma (B NHL) in children was significantly higher (P less than 0.002) than the LI of untreated T cell malignancies (median LI 27.7% and 10.3%, respectively). In B NHL abdominal tumour material showed a significantly higher in vitro incorporation of tritiated thymidine (3H-dT) than extra-abdominal tumour material (P less than 0.01). Sequential immunological, cytochemical and cytokinetic studies revealed tumour cells in up to three marker-defined subpopulations of lymphoid cells. In all 14 patients examined the LI of E-Ia+sIgM+ cells was higher than the LI of E+Ia+/-sIgM- cells; in all eight patients with Ia+ and Ia- tumour cells the former had a higher LI than the latter (P less than 0.002). In all T cell neoplasia (T ALL/NHL) investigated malignant cells were found in two of the three marker-defined subsets of lymphoid cells. In 10/13 patients the E-rosette positive blast cells had a higher LI than the E-rosette negative T blast cells. For the small group of patients with T cell malignancies no significant difference in the overall LI between OKT9+ stage I (early thymocyte) and OKT9- stage II (cortical thymocyte) neoplasias could be found. However, within the malignant clone of a T cell malignancy the expression of the OKT9 (transferrin receptor) but not the cortical thymocyte (HTA-1) antigen was related to the in vitro 3H-dT incorporation.