Tetracycline resistance in clinical isolates of Campylobacter jejuni was shown to be plasmid mediated. Intra- and interspecies transfers to C. fetus subsp. fetus were demonstrated. The frequency of transfer was increased by approximately 100-fold on a solid surface by using a plate- or filter-mating procedure, as compared with a liquid-mating method. Results of experiments in which cell-free filtrates were used to replace the donor strain in mating experiments tend to rule out bacteriophage-mediated transduction in the transfer of tetracycline resistance. The plasmid-transfer frequency was not affected when deoxyribonuclease was added to the agar used in the mating experiments, indicating that transformation was not involved. Four transmissible plasmids from different tetracycline-resistant strains of C. jejuni each had a molecular weight of 38 x 10(6). Transfer of these plasmids to Escherichia coli was not demonstrated.