&NA;Recombinant human interferon beta (rIFN‐&bgr;) reduces replication of HIV in cultured peripheral mononuclear cells. The effect is most pronounced when high levels of the drug are employed. Maintenance of the rIFN‐&bgr; in culture is required since removal of the agent generally leads to a return of virus production by the infected cells. Moreover, at low concentrations of the drug, a breakthrough in HIV replication is observed. High concentrations of the rIFN‐&bgr; (> 100 units/ml) were cytotoxic for transformed T cells. This latter observations suggests that rIFN‐&bgr; might be useful in human T‐cell malignancies. Beta interferon therefore might be useful for the treatment of HIV infection, particularly since side effects of the drug are limited in treated individuals.