In extending the scope of our previous analyses of tiie functions that mammalian cells engender to recombine elements of genetic information (Upcroft, Carter and Kidson, 1980a and b), I describe the capacity of cells to recognise differentially combinations of open-ended and closed molecules. The frequency of successful recombination, as assayed by the generation of viable SV40 genomes from appropriate substrates, was shown to decrease with the loss of free termini capable of invasion of an homologous DNA duplex and to be influenced by the presence of non-homologous flanking sequences, the length of homology and whether the sequence was internal or terminal. The effect of stabilisation with long homologous regions on recombination over short homologous regions and inter- versus intramolecular DNA sequence homologies was also investigated.