The objective of this study was to evaluate the transmission of M. hyopnemoniae and PRRS by aerosol as either a single or mixed infection. 28 pis were inoculated with M. hyopnemoniae and with PRRS 35 days later. These pigs were all held together in the same barn. To test for single infection (before day 35) a group of 10 five week old pigs were put outside the barn in a trailer one meter away from the fan. To test for mixed infection two groups of 10 week-old pigs were placed on the side of the barn after the pigs had been infected with PRRS. The pigs in the first trailer did not test positive for M hyopneumoniae infections. In the other two trailers pigs tested positive for a M hyopneumoniae but none tested positive for PRRS.