The site-specific integration of exogenous gene fragments by homologous recombination provides a convenient method for altering the immunoglobulin loci of B cells and specifically designing antibody molecules. To introduce a human isotype into the heavy chain locus of mouse hybridoma cells we compared the recombination frequencies of vectors that could be linearized either as integration or as replacement constructs in different cell lines. Integration as well as replacement recombination was observed, irrespective of the location of the site at which the vector was cleaved. Integration events involving the human IgG1 vectors were lost at high frequency due to secondary vector excision, so that all stable recombinations were found to be replacement events. Replacement recombination of an integration vector involves an illegitimate crossover at least at the 3' side and sometimes gives rise to deletion of the CH1 domain. However, a homologous event at the 3' side is more efficient than an illegitimate one, so that a homology that is distributed on both sides of the heterologous region promotes targeting at higher frequency than a contiguous sequence of the same total length. The position of the linearization site in the vector markedly influenced the targeting efficiency, but surprisingly, whether a double-strand break in the homology or in the heterology region more efficiently promoted integration was dependent on the cell line. In all cells, however, cleavage of the vector outside the homology region favoured stable replacements with a bias against CH1-truncated clones. We further show that the frequency of replacements induced by integration vectors is not correlated to the homology length and cannot be increased by irradiation of the cells. Our findings indicate that for targeting the IgH locus other mechanisms might be involved than at other loci.