Propionic and acetic acid production by Propionibacterium cells immobilized in calcium alginate beads was evaluated in 12-h repeated-batch fermentations on glucose and lactate as primary carbon sources. Beads contained approximately 2 x 10(11) cells g(-1) (wet weight) and were added at 40% of broth volume. High initial substrate concentrations (up to 115 gl(-1) glucose and up to 78 gl(-1) lactate) were utilized by the immobilized cells. The best substrate utilization rates and acid yields were obtained al 75 gl(-1) initial glucose and 42 gl(-1) initial lactate; 34 gl(-1) and 22 gl(-1) propionate were produced respectively, at these substrate concentrations in 12 h. The ratio of propionic to acetic acid increased to 9:1 with increasing glucose concentration, bur remained at 2:1 at the different initial lactate concentrations tested. Glucose consumption and propionate production rates were higher in fermentations controlled at pH 6.0 than at pH 7.0, but yields of acids from glucose were highest at pH 7.0.