1. The antiviral effect of isoprinosine on simian rotavirus (SA-11) replication was studied using MA-104 cell cultures from Rhesus monkey fetal kidney. 2. Isoprinosine (N,N-dimethylamino-2-propanol-p-acetamidobenzoate in association with inosine) added after viral infection (therapeutic test) inhibited viral replication by more than 90%. In these experiments, the drug was added to the medium and replaced daily at concentrations varying from 62.5 micrograms/ml to 1 mg/ml. Viral inhibition activity was dependent on drug concentration. No antiviral effect was observed when isoprinosine was tested without replacement (200-500 micrograms/ml). 3. When isoprinosine (1 mg/ml) was added to cell cultures only before viral infection (prophylactic test), inhibition of viral replication occurred but was less than 90%. Inhibition by less than 90% is not considered to be significant in this type of test. 4. Isoprinosine inhibited synthesis of both viral antigen (protein) and viral double-stranded nucleic acid, as monitored by immunofluorescence and acridine orange staining, respectively. Inhibition of synthesis of viral macromolecules increased with drug concentration.