The relationship between the uptake of glucose and glutamine and the partitioning of these substrates into hybridoma growth and monoclonal antibody production at steady state in a chemostat was investigated. The mouse-mouse hybridoma cell line 6BB which produces IgM against human blood group B was used. It was observed that the specific uptake rate of the rate-limiting substrate (Q(s)) was determined by the residual concentration of the substrate in the culture (R-s); however, the specific growth rate of the culture was not strictly determined by R-s as described by the Monod Relation nor determined by Q(s). The specific rate of production of monoclonal antibody (Q(ab)) was a function of dilution rate and the concentration of the rate-limiting substrate in the medium reservoir. Simple mathematical models were developed which suggested that the rate-limiting substrate taken up by the cultures was channeled either into biomass or antibody production. Partitioning og the rate-limiting substrate was a result of competition between the cell growth and the antibody production mechanism.