Recent human isolates of Trypanosoma brucei gambiense generally fail to become or remain patent in laboratory rodents. The purpose of this study was to determine if this was due to acquired immunity and if so which immunosuppressive method was the most efficient in raising parasitemia levels. Prior to infection, rats and mice were immunosuppressed by treatments with cobra venom factor, anti-lymphocyte sera, hydrocortisone acetate, cyclophosphamide; by splenectomy; or by lethal X-irradiation. While no parasites were detected in the blood of most of the untreated rodents for 30 days postinfection, all immunosuppressive procedures resulted in patent parasitemias in at least fifty percent of the treated animals. The most effective method, lethal X-irradiation, consistently caused fulminating infections typical of acute African trypanosomiasis. Cyclophosphamide had the same effect as X-irradiation in rats but was less effective in mice. Splenectomy allowed fulminating first peak parasitemias in two-thirds of the rodents while cobra venom factor and anti-lymphocyte sera in general allowed only low first peaks of parasitemia that were resolved within 10 days of infection. Hydrocortisone acetate allowed low grade and sporadically patent infections throughout the 30-day study. To determine if in untreated rodents, the parasites were eliminated or maintained in a subpatent state, rodents infected for 30-45 days were immunosuppressed with cyclophosphamide.(ABSTRACT TRUNCATED AT 250 WORDS)