Single lymph node cells from rats immunized with a conjugate of 3-iodo-4-hydroxy-5-nitrophenyl acetic acid chloride (NIP) and chicken globulin were tested against either NIP or NNP conjugated with bacteriophage. NNP is related to NIP but has a nitro group instead of iodine. The conjugated phages were still infective but easily inactivated by anti-hapten sera at a serum concentration of 10(-7). ...