Twenty patients with severe aplastic anemia (SAA) were treated with low doses (1-5 mg/kg/day) of a high-potency antithymocyte globulin (ATG) produced in Mexico, shown to have at least a 10-fold potency as compared with other globulins of commercial sources. Patients received ATG within a 10-day period, every other day (5 doses) at a dose of 1 mg/kg/day (4 courses), 2 mg/kg/day (12 courses) or 5 mg/kg/day (8 courses). Four patients received 2 consecutive courses of different doses of ATG. A response rate of 42% was recorded in the group, assessed by means of increases in reticulocytes, granulocytes or platelets. One patient showed a complete remission. The 570-day survival of the group was 51%. It is concluded that the domestically produced ATG is useful in the treatment of some patients with SAA in Mexico.