The impact that the continuous presence in the fermentation broth of the cognate antigen has on the serum-supplemented hybridoma cell cultures was investigated. Both soluble and immobilized antigen at various concentrations was applied. The cell line (ATCC TIB191) was cultured in a serum-supplemented PFHM-II medium in T-flasks. Sepharose gel beads provided the immobilization matrix, and bovine gamma-globulin was the carrier protein upon which the antigen, picric acid, was conjugated. Produced antibody, after elution from the beads by displacement with free picric acid, was measured with an ELISA. Soluble antigen-carrier protein conjugates showed no effect on the cultures, but the immobilized antigen had a strong influence on them. Cell growth rate and total antibody production decreased as the amount of immobilized antigen increased from 16:10 (units are in (mol of Ag/mol of carrier):(mg of carrier/mL of beads) to 36:40. Most importantly, the specific antibody production rate switched from growth phase-independent behavior in the antigen-free cultures to growth phase-dependent behavior in the immobilized antigen cultures. Furthermore, during the early stationary phase of the cultures with immobilized antigen, the specific antibody production was 20-100% higher relative to the production rate in antigen-free cultures. This increased specific antibody production rate would be particularly useful in increasing the volumetric productivity of perfusion-type bioreactors for hybridoma cell cultures.