The physiology of growth and monoclonal antibody (Mab) production by a murine hybridoma, PQX B1/2, which secretes an antibody to paraquat, has been investigated. A triphasic sequence characterising antibody production was revealed by partial cubic spline interpolation of measurements of culture biomass, Mab titre, glutamine, ammonia and amino acids concentrations. During phase I peaks in the utilisation of amino acids, primarily GLN, and ammonia production, were revealed together with apparent inhibition of growth. Specific growth rate peaked in phase II, during which ammonia was apparently assimilated. This assimilation was investigated, and the mechanism proposed to be via a cyclic coupling of glutamate dehydrogenase to alanine aminotransferase. Further peaks in the assimilation of amino acids were observed in phase III during which Mab production rate reached a maximum. These observations are discussed in relation to possible cell-cycle events. The processes of growth and product formation were further investigated by determination of materials balance equations. The nutritional requirements for each phase were found to be different. The significance of amino acids for hybridoma culture has been highlighted, and a fractional factorial method employed to investigate the importance of individual amino acids for growth and production. The requirements for each process were found to be different: some amino acids were important for growth but not critical for Mab production (eg lysine) whereas others (eg serine) could be omitted with little effect on growth but a profound effect on product formation.