Acid production of lactic starter cultures was stimulated by addition of cell-free filtrates from milk cultures of P. fluorescens. The average increase in titratable acidity, using 1% added filtrate, was 23% in 14 hours at 21[degree] C. Autoclaving the filtrate greatly reduced stimulation but boiling for 5 minutes caused only slight reduc -tion. The filtrate partially counteracted the inhibitory effect of small amounts of penicillin or chlortetracycline on acid production of lactic cultures. It appeared that stimulation arose partly from protein fractions and partly from enzyme activity.