A laboratory scale bioreactor was used for continuous acidification and inoculation of milk with a proteinase-negative, lactose-fermenting strain, Lactococcus lactis subsp. lactis C2S. Calcium alginate-entrapped cells were immobilized on a spiral stainless steel mesh incorporated into a column bioreactor and used to acidify and inoculate reconstituted skim milk. Characteristics of the immobilized cell bioreactor (ICB) were compared with those of a free cell bioreactor (FCB) during challenge with a virulent phage. Steady state biomass and lactate productivities were respectively 25-fold and 12-fold larger with the ICB than with the FCB. The ICB and the FCB were inoculated with the prolate phage c2 at multiplicities of infection of 0·25 and 0·02 respectively. Within 90 min of the infection, the FCB viable cell concentration dropped by five orders of magnitude and never recovered, while the plaque forming units/ml increased dramatically. In the ICB, released cells decreased immediately after infection, but subsequently increased, while the plaque forming units/ml steadily declined, indicating that phage were being washed out of the bioreactor. Productivity of FCB decreased to zero, whereas productivity of the ICB only decreased ∼ 60% and subsequently recovered to its initial steady state value.