BACKGROUND: Therapies using Y-anti-CD20 or I-anti-CD20 have demonstrated their efficacy in patients with B-cell non-Hodgkin's lymphoma. Re is a radionuclide useful for radioimmunotherapy. AIM: To develop a procedure for efficient labelling of anti-CD20 with Re from lyophilized formulations to achieve high radiochemical yield, high specific activity and preservation of the molecular recognition after a simple kit reconstitution without further purification. METHODS: Re-anti-CD20 was prepared by a direct labelling method using sodium tartrate as a weak competing ligand. Different lyophilized formulations were prepared to optimize tartrate and stannous chloride concentration, pH and reaction time. To evaluate the biological recognition a comparative study of the in-vitro binding of Re-anti-CD20, I-anti-CD20 (positive control) and Re-anti-CEA (negative control) to normal B lymphocytes was performed. Biodistribution studies in normal mice were accomplished to assess the in-vivo Re-anti-CD20 complex stability. RESULTS: Re labelled anti-CD20 was obtained with high radiochemical purities (>97%) and high specific activity (0.5-0.7 GBq . mg) 1-1.5 h after addition of sodium perrhenate solution to a kit containing 4.4 muM anti-CD20, 4 mM anhydrous stannous chloride, and 140 mM dihydrate sodium tartrate at pH 4. The binding of Re-anti-CD20 to cells was in the same range as I-anti-CD20 (>80%) and was significantly different to cell binding of Re-anti-CEA (<10%). No evidence of free Re release was found at 2, 4 and 24 h after Re-anti-CD20 administration in mice. Lyophilized kits showed high stability during the storage at 4 degrees C for 6 months. CONCLUSIONS: Optimal reaction conditions were defined enabling high radiochemical purities of Re-anti-CD20 to be obtained routinely and therefore potentially useful in the treatment of non-Hodgkin's lymphoma.