The antiretroviral activity, tolerance and toxicity of two different antiviral drug combinations were assessed and compared in a randomized, crossover pilot study in 16 HIV-1 p24 antigenaemic subjects with asymptomatic HIV infection. Oral zidovudine 250 mg twice daily was combined with either oral acyclovir 800 mg twice daily or lymphoblastoid interferon-alpha 1.5 x 10(6) IU administered subcutaneously three times weekly. The 12-week treatment period was followed by a 4-week washout period and a further 12-week crossover phase. During the entire treatment period a decline in p24 antigen was observed in all patients. No significant differences were found between the two treatment regimens. No patient showed clinical progression of HIV infection. Three patients were withdrawn from the study, one due to serious anaemia and two due to severe clinical adverse events. Long-term efficacy and tolerance data in asymptomatic HIV-infected patients with these regimens would be valuable.