The influence of four antifoam agents (AFA) : S184 (pure silicon oil), SLM54474 (polypropylene glycol; PPG), VP1133 (silicon oil/PPG mixture), and SE9 (aqueous emulsion with 10% silicon oil component) of Wacker-Chemie Co., was determined on the foaminess of cultivation media of recombinant Escherichia coli, Penicillium chrysogenum and Cephalosporium acremonium. The effect of these AFAs was evaluated during the cultivation of recombinant E. coli on the following process variables : gas holdup, bubble velocity, oxygen volumetric mass transfer coefficient (K(L)a), cell mass concentration (X), specific growth rate (mu), product (fusion protein) concentration (P), specific product activity (SP), plasmid stability, number of colony-forming units (CFU), specific glucose uptake rate (SG), acetate production rate, oxygen transfer rate (OTR), CO2 production rate (CPR) and respiratory quotient (RQ) The antifoam agents in the following initial concentrations, 100 (VP1133), 300 (SLM54474), 1000 and 2000 ppm (S184) were effective in foam depressing during batch cultivation off. coli in small reactors and had a slight or positive effect on process performance Their presence enlarged X and CFU. The addition of SLM54474 and/or VP1133 increased P and SP, and slightly reduced Lc. Addition of S184 reduced P and SP but did not influence mu. In large reactors SE9 and VP1133 proved to be the most efficient in foam suppression, but pure silicon oil (S184) and pure PPG (SLM54474) were inefficient.