SummaryThe safety of continuous i.v. interleukin-2 (IL-2) in conjunction with zidovudine (ZDV) was assessed in asymptomatic patients infected with human immunodeficiency virus. Clinical, immunologic, and viral parameters were monitored in a phase I/II trial with dose escalation and crossover arms. Daily doses of IL-2 from 1.5 to 12 x 106IU/m2were well tolerated and, in the presence of ZDV, did not induce increases in p24 antigenemia. Significant (p< 0.05) but transient increases in CD4 cells were observed midway through infusion of IL-2 at all doses, and increases in natural and lymphokine-activated killer activity were seen at higher doses. Circulating hypodense eosinophils and soluble IL-2 receptors increased more than 10-fold. Of nine patients available for long-term follow up 13–25 months from baseline and 4–21 months after stopping IL-2, six still had improved CD4 counts (versus baseline), and the mean increase (135/mm3) for all nine patients was significant (p< 0.05). Eight of these nine patients were negative for serum p24 at the start of therapy, and none had become p24 antigenemic at long-term follow-up.