Particle-mediated (Helios(R) Gene Gun) transfer to the turkey epidermis of plasmid DNA expressing the major outer membrane protein (MOMP) of an avian Chlamydia psittaci strain was evaluated for its ability to raise an immune response and protection against challenge with the homologous strain. In turkeys, the delivery of pcDNA1/MOMP coated onto 0.6 mu m Sold beads was the most efficient compared to immunisations using 1.0 or 1.6 mu m gold beads. The delivery of as little as 1 mu g; pcDNA1/MOMP coated onto 0.6 mu m gold beads was efficient. Immunisation with 1.0 mu m Sold beads required twice more (2 mu g) DNA to achieve comparable results. The use of 2 mu g DNA coated onto 1.6 mu m gold beads had no effects. The gene gull delivery both primed T-helper and B-cell memory although recombinant MOMP-expressing cells did not induce high-titre antibody responses. The significance of gene gun-based DNA immunisation as a means of preventing severe clinical signs, lesions and chlamydia excretion in a turkey model of Chlamydia psittaci infection was demonstrated. (C) 1999 Elsevier Science Ltd. hu rights reserved.