Chromosome transfer occurred in plate matings between Proteus mirabilis strain PM5006 and Providencia alcalifaciens strain P29 in either direction with the use of plasmid D or R772 as sex factor. Auxotrophic chromosomal markers of recipients were converted to prototrophy and the galactose fermentation marker of donor PM5006 could also be selected. Recombination frequencies for a group of selected markers in PM5006(D) x P29 matings varied between 3 x 10(-5) (trp+) and 1.2 x 10(-7) (lys+) per donor. In the reverse cross, plasmid D mobilized markers on the P29 chromosome randomly with a recombination frequency of about 1.7 x 10(-7) per donor for all selected P29 markers. R772 produced random mobilization of markers on both chromosomes yielding recombinants at a frequency of about 1.8 x 10(-7) per donor. Unselected markers separated by no more than about 10 min from selected markers on the PM5006 chromosome were cotransferred from P29 by both plasmids. Despite the low degree of DNA homology existing between the two species, all hybrids behaved as stable haploids. Progeny from P29(D or R772) x PM5006 auxotroph matings displayed similar sets of naturally occurring P29 unselected markers irrespective of the selected prototroph allele. In reverse crosses, a similar range of PM5006 naturally occurring unselected markers registered in P29 recipients, although differences existed in the sets of markers mediated by the two plasmids. Weak linkage was detected between PM5006 gal+ allele(s) and some P29 auxotroph markers. Adsorption of donor-specific phages 5006M or PL25 to hybrids could not be demonstrated and many recombinants failed to express some or all of the plasmid markers.