Two markers for cells in the growth fraction, the T9 antigen (i.e. the transferrin receptor) and the T10 antigen were investigated in frozen tissue sections of 105 non-Hodgkin lymphomas (NHL). The results were correlated with the histological subtype and the pattern of tumour infiltration by reactive cells. Special attention was directed to the density of natural killer (NK)-like cells using the anti-HNK1 (Leu7) antibody since the transferrin receptor (tfr) or other growth-associated membrane structures may serve as target for NK cells. Our study confirms a relationship between number of tumour cells with the T9 marker and tissue infiltration by HNK1+ cells in NHL of low (chronic lymphocytic leukaemia, hairy-cell leukaemia, immunocytic lymphoma, centroblastic-centrocytic lymphoma) and intermediate (centrocytic and centroblastic lymphoma) but not in NHL of high malignant grade (immunoblastic and lymphoblastic lymphoma). Comparable results were obtained with the T10 antigen although the correlation was less close. The percentage of cells in the growth fraction, defined by the expression of the T9 and T10 marker, corresponded with prognostically unfavourable subgroups with the remarkable exception of follicular NHL of centroblastic-centrocytic type. This lymphoma showed high numbers of cells with the T9 and the T10 marker in a microenvironment resembling normal germinal centres in many aspects.