Derivatives of phage lambda are described for use as vectors for fragments of DNA generated with the HindIII and EcoRI restriction enzymes. With some vectors, hybrid molecules are recognised by a change from a turbid to a clear plaque morphology resulting from the insertion of a fragment of DNA into the lambda gene coding for the phage regressor. Other vectors contain a central, replaceable fragment of DNA which imparts a readily recognisable phenotype. This central fragment may include either a gene for a mutant transfer RNA (suppressor) or a part of the lacZ gene of E. coli able to complement a lacZ host. The appropriate lacZ host and indicator plates permit the ready distinction between recombinant and vector phages by the colour of the plaques.