R18-1 and R91-1, two antibiotic resistance factors (R factors), specifying resistance to carbenicillin, and RP1, specifying resistance to carbenicillin, neomycin, kanamycin, and tetracycline, were transferred from Pseudomonas aeruginosa to P. syringae.  RP1 was also transferred to P. phaseolicola and P. glycinea.  R18-1 was transferred to these three species and to P. pisi, P. marginalis, P. putida, and P. fluorescens.  The mechanism of transfer of R18-1, and presumably of the other R factors, was conjugation.  The transfer frequency per donor cell was 10^–6 to 2 × 10^–3 in the case of R18-1, 3 × 10^–7 to 2 × 10^–5 for R91-1, and 5 × 10^–4 to 2 × 10^–2 for RP1.  Acquisition of the R factors conferred upon the recipients capacity to produce penicillinase, ability to act as donors for antibiotic resistance, and (in the case of recipients of RP1) susceptibility to phage PRR1.  Neither hypersensitive reaction on tobacco, nor pathogenicity of three species that were tested, was affected by inheritance of the R factors.  Spontaneous segregation of R91-1 occurred at high frequency, and of R18-1 and RP1 at low or zero frequency, which in some cases was increased by sodium dodecyl sulfate, indicating a plasmid condition of the R factors in the plant pathogens.  Acceptance of these and other P-group plasmids makes possible the search for a system for conjugative chromosome transfer in plant pathogenic Pseudomonas spp.