An EBV-transformed human B cell line, which produces monoclonal IgM antibodies, was cultured in an immobilized state on a ceramic matrix in the Opticell system. Cell growth, metabolic activity and product formation of these cells were optimized in a single semi-continuous culture of 10 inductions for 29 d. All in all, 19.5 g of IgM were harvested in 193 l of culture supernatant. The average production of IgM in the Opticell unit of 20 l was 0.7 g/d which was obtained even in serum-reduced medium (1%). Under optimal conditions IgM yields were enhanced to 1.5-2.0 g/d. These results indicate that the Opticell is a suitable system for the large scale production of monoclonal antibodies, particularly because the capacity for aeration and pumping of one Opticell unit is sufficient to control 3 growth chambers running in parallel.