An interferon beta polypeptide comprising interferon-beta 1a coupled to a polymer containing a polyalkylene glycol moiety wherein the interferon-beta-1a and the polyalkylene glycol moiety are arranged such that the interferon-beta-1a has an enhanced activity relative to another therapeutic form of interferon beta (interferon-beta-1b) and exhibits no decrease in activity as compared to non-conjugated interferon-beta-1a. The conjugates of the invention are usefully employed in therapeutic as well as non-therapeutic, e.g., diagnostic, applications.