Anti-idiotypic antibodies are being studied as possible high-affinity probes for virus-receptors. Antibodies have been elicited in rabbits by immunization with Fab fragments of monoclonal antibodies against coxsackievirus B4, and the antiFab antibodies are being tested to determine if they posses anti-receptor activity. Monoclonal antibodies against coxsackievirus B4 have also been used to select antigenic variants of this virus and to measure the frequency of variation in laboratory grown and clinically isolated viruses. Hormone-producing hybrid cells have been made by fusing rat pancreatic islet cells with established murine fibroblast and myeloma cell lines. Clones of these hybrid cells secrete hormones for at least several passages in vitro.