Microgravity can influence cell growth and function. A transfected Sp2/0 myeloma cell line P3A2 producing a human IgG1 anti-TNFalpha monoclonal antibody was cultivated in static culture, spinner flasks and simulated microgravity using a rotating wall vessel bioreactor. Microgravity significantly decreased cell growth ( from 1.7 x 10(6) to 7.9 x 10(5) cells/ml), but facilitated the synthesis of antibodies, (1.8, 1.3 and 0.5 mug of anti-TNFalpha hmAb per 10(6) viable cells for cells cultivated under microgravity, in spinner flasks and static cultures, respectively). The results suggest that microgravity could be applied to improve the specific productivity of cell lines producing potentially important therapeutic proteins.