Single-strand interruptions in a template DNA are likely to cause collapse of replication forks. We propose a model for the repair of collapsed replication forks in Escherichia coli by the RecBCD recombinational pathway. The model gives reasons for the preferential orientation of Chi sites in the E. coli chromosome and accounts for the hyper-rec phenotype of the strains with increased numbers of single-strand interruptions in their DNA. On the basis of the model we offer schemes for various repeat-mediated recombinational events and discuss a mechanism for quasi-conservative DNA replication explaining the recombinational repair-associated mutagenesis.