Antibodies to the canine nasal mite, Pneumonyssoides caninum, were demonstrated by ELISA in sera from four experimentally and 77 naturally infected dogs employing a crude P. caninum antigen. In sera from the four experimentally infected dogs, antibodies to P. caninum were first detected on day 11 post-inoculation (p.i.). Optical density (OD) values remained high throughout the observation period of 14 weeks. Two major protein bands with apparent molecular weights of 83 and 74kDa were visible by western blot (WB) from day 11 p.i. On day 14 p.i., another three major protein bands at 44, 37, and 34kDa were visible. The protein pattern then remained essentially constant until the end of the experiment except for some weak bands between 250 and 30kDa which were sometimes visible. In sera obtained from a non-inoculated control dog, no P. caninum antibodies were detected, neither by the ELISA nor by the WB. With sera from 77 naturally infected dogs with a verified diagnosis of P. caninum infection, the OD values varied between 0.24 and 0.95. When sera from four of these dogs were analysed by WB, the protein pattern observed was equivalent with that seen with sera from the experimentally infected dogs on day 25 p.i. Sera from four experimental Beagles, in which no P. caninum mites were found at necropsy, were also analysed by the P. caninum ELISA, the OD values demonstrated never exceeded 0.15. In addition, 48 sera obtained from dogs infected with or allergic to other arthropods were used for studies of possible cross-reactivity. With 10 sera, elevated OD values were found by the ELISA. Eight of them showed a similar banding pattern by the WB as did the experimentally infected dogs on day 25 p.i. This was, therefore, interpreted as an effect of a concurrent P. caninum infection.