A previously-described fused plasmid, P-lacRIdrd19, was found to mediate chromosomal transfer between cells of Proteus mirabilis strain PM5006; PM5006-(P-lacRIdrd19) was usually the donor and various auxotrophs of PM5006 resistant to nalidixic acid and/or streptomycin were recipients. The donor was usually counterselected with nalidixic acid and/or high concentrations of streptomycin. Recombination experiments with single markers indicated a 40-fold variation in recombination frequencies for different markers. Mapping double-auxotrophic markers by their gradient of transmission confirmed this variation and placed each of two independent isolates of eight markers in a linkage group his-ser-ura-pyrB-trp-cys-ade-ilv. Some donor markers did not register. Despite low recombination frequencies, interrupted mating experiments showed a polarity of early marker transfer. The segregation of unselected markers confirmed the order of some markers and showed that genetic material passed from the presumptive donor to the recipient. Recipients with two auxotrophic markers which could not be cotransduced by phage 5006M were converted to prototrophy by conjugation. The plasmid transferred to recipients at high frequency and all recombinants carried it. Recombinants could act as donors in further matings. Recombinants were fully susceptible to phage 5006M, unlike transductants of PM5006 by this phage. Direct involvement of the plasmid was indicated by drastically diminished recombination frequencies in crosses with recipients carrying P-lac as resident. P-lac had previously been shown to reduce the frequency of transfer of the hybrid plasmid to cells harbouring it. The histidine region was the first to register in recipients and recombined at the highest frequency of 5 times 10-6/donor cell. Some temporary association of plasmid and perhaps only the histidine region of the chromosome is favoured as the mechanism of chromosomal transfer. This could explain why not all donor markers could be mapped. Transduction and transformation were excluded as the cause of results.