The sera of 135 patients being treated with Human leukocyte alpha interfero (Hu IFN (Le)(New York Blood Center, Melville, N.Y.) for Juvenile Laryngeal Papillomatosis (J.L.P. ) were tested for both binding and neutralizing antibodies to Interferon. Sera were screened for binding antibodies against Interferon alfa 2b, Alferon (Interferon alfa-n3) and Wellferon (a preparation of human lymphoblastoid IFN-alphas from Wellcome Research Laboratories, Beckenham, England). Six samples, which were found to be positive for binding antibodies on all of the abovementioned interferons, were also tested for neutralizing antibodies to Leukocyte interferon. Two of the samples positive for binding antibody to interferon were assayed for their possible cross-reactivity to six of the human lymphoblastoid IFN- alpha species. In the future, more J.L.P. samples will be tested for both binding and neutralizing antibodies and any positives will be assayed against as many lymphoblastoid species as possible.