Over sixty EMS induced mutations affecting gene lamB, presumably the structural gene for the lambda receptor in Escherichia coli K12, were examined for growth of lambda host range mutants and effect of nonsense suppressors. By the first criterion the mutations could be grouped in three classes. Bacteria with class I mutations allow growth of lambda mutants with extended host range (noted lambdah) of the type already described (Appleyard, MacGregor and Baird, 1956). Bacteria with class II mutations allow growth of lambdah mutants with still more extended host range (noted lambdahh). No host range mutants of lambda could be found which would grow on bacteria with class III mutations. Using nonsense suppressors it was found that class I and II consist of missense mutations, while class III consists of nonsense mutations. Exceptions are likely to exist (especially in class III) but were not found among the mutations tested. These observations are briefly discussed in terms of outer membrane protein integration and of phage receptor interaction.