The objective of this study was to determine the antimicrobial activity of pepsin-digested lactoferrin added to carrot juice and filtrates prepared from carrot juice. Lactoferrin isolated from raw skim milk was digested by pepsin for 4 h at pH 3. The digest of lactoferrin was lyophilized, and the antimicrobial activity of the digests was determined in peptone-yeast-glucose broth, carrot juice, permeate from carrot juice, and the dialysate of carrot juice permeate using Escherichia coli (American Type Culture Collection strain 35343) as the test organism. Growth of E. coli and the inhibitory effect of the peptide were greater in peptone-yeast-glucose broth at pH 7 than at pH 4. The peptic digest of lactoferrin did not have antimicrobial properties in carrot juice at concentrations of less than 10 mg/ml of juice. Carrot juice was filtered through a membrane with a molecular weight rejection of 10,000 or 500 Da, and the permeate was dialyzed against distilled water. Growth of E. coli was delayed in the filtrate by 5 mg but not by 1 mg of the peptic digest of lactoferrin per ml of filtrate. Bacterial counts of the control and experimental samples were not significantly different after 24 h of incubation. The peptic digest of lactoferrin at a concentration of 5 mg of digest per ml of dialysate was bacteriostatic toward E. coli after 24 h of incubation at 23 degrees C. Dialysis of permeate caused a percentage reduction in cation concentration in the permeate ranging from 69.23% (Co) to 99.32% (Na). The antimicrobial activity of lactoferrin added to carrot juice was probably inhibited by cations.