OBJECTIVES:The effects of two humate products were compared to that of prednisolone on a contact hypersensitivity rat model. METHODS: Rats, sensitized with dinitrofluorobenzene (DNFB), were placed on a daily oral treatment of 61mg/kg BW of humate derived from either leonardite or bituminous coal or on prednisolone at one mg/kg BW and challenged 6 days later with a topical application of DNFB to the right ear. The inflamed ears were measured daily. In a toxicity study rats were exposed to daily oral treatment of leonardite humate at 1,000 mg/kg BW for 1 month. A teratogenicity study was done where pregnant rats were treated with 500 mg/kg BW on days 5 to 17 of pregnancy. RESULTS: Only the leonardite humate compared favourably with prednisolone in suppressing contact hypersensitivity. No signs of toxicity were observed and weight gain was normal during the 6-day and 1month treatments and during the teratogenicity study with the leonardite humate. However, the rats on the other two products experienced slower weight gain. CONCLUSION: The identification of a naturally occurring nontoxic compound with anti-inflammatory activity is exciting and merits further evaluation in the treatment of patients suffering from inflammatory conditions.