The levels of deoxythymidine kinase in tumour cells (C-TK) and in serum (S-TK) were investigated and the tumour volume calculated in 89 patients with non-Hodgkin lymphoma (NHL), in order to ascertain the importance of C-TK and tumour burden as regards the S-TK levels. The gene discussed is TKT; the disease is non-Hodgkin lymphoma.