This study describes a prospective, randomized, clinical trial in patients infected with the protozoa Giardia lamblia. Patients received a 10-day treatment with twice a day doses of either 120,000 U (USP) of bacitracin zinc, 120,000 U (USP) of bacitracin, 120,000 U (USP) of neomycin, or 60,000 U (USP) of bacitracin zinc and 60,000 U (USP) of neomycin. At the first assessment (day 11), all 21 subjects (100%) treated with bacitracin zinc had ceased to show Giardia parasites in their stools compared with 19 (95%) of 20 receiving bacitracin, 20 (90.9%) of 22 subjects receiving neomycin, and 17 (89.5%) of 19 subjects receiving bacitracin zinc plus neomycin. During the two-week follow up period, one (5.3%) of the 19 subjects examined who received bacitracin zinc experienced a recurrence compared with one (6.7%) of 15 receiving bacitracin, one (5.0%) of 20 receiving neomycin, and 0 (0%) of 14 receiving the combination treatment. Final cure rates of 94.7% for bacitracin zinc, 87.5% for bacitracin, 86.4% for neomycin, and 87.5% for bacitracin zinc plus neomycin were obtained. No synergistic activity was noted between bacitracin zinc and neomycin. Side effects were generally limited to nausea, abdominal discomfort, and diarrhea in a small number of patients.