B-cell chronic lymphatic leukaemia (B-CLL) is characterized by proliferation and accumulation of small B-lymphocytes, which are monoclonal in organ. The changes in IL-6 and IL-12 concentrations usually occur during the course of B-CLL. IL-6 and IL-12 seem to be positive kinetic regulators of stem cells. Therefore the purpose of our study was to examine the changes in concentrations of IL-6 and IL-12 in blood plasma, culture supernatant and isolated and broken lymphocytes from patients with B-CLL. The study was performed in I (n = 12) and III (n = 12) stage of disease according to Rai's classification--20 males and 4 females (aged 45-65) and in 12 healthy volunteers blood donors 35-55 years old. The study was approved by the local ethics committee. The measurement of concentrations of IL-6 was performed using the IL-6 immunoenzyme set of ELISA, R&D Systems Europe (UK) in plasma, culture supernatant and broken leukaemic cells. The results showed a significant increase in IL-6 and IL-12 concentration in blood plasma, culture supernatant and inside of the lymphocytes at I and III stage of B-CLL with regard to control groups. An increase of IL-6 and IL-12 concentrations in blood plasma and culture supernatant may suggest higher secretions by lymphocytes these interleukins during the course of B-CLL. An increase of IL-12 in broken leukaemic cells could be characteristic for the biochemistry of malignant lymphocytes.