Effect of prior porcine respiratory coronavirus (PRCV) infection on replication of H1N1-influenza virus in the respiratory tract of swine was studied. In an initial experiment, 3 groups of 5 feeder pigs were studied. Pigs of 2 groups were inoculated sequentially with PRCV, followed by H1N1-influenza virus at 2- and 3-day intervals. Pigs of the other group were inoculated with H1N1-influenza virus only. Pigs were monitored clinically and examined for nasal excretion of influenza virus. In the singly influenza virus-inoculated group, 83% of nasal swab specimens were influenza virus-positive over a period of 6 days after inoculation. In the dually virus-inoculated groups, only 27% (2-day interval) and 53% (3-day interval) of nasal swab specimens were virus-positive over the same postinoculation period. However, clinical signs of infection in these dually inoculated pigs were more severe than those in the singly influenza virus-inoculated pigs. There were no significant differences in antibody responses against influenza virus among the 3 groups of pigs. In a second experiment, 2 groups of pigs were studied. One group of pigs was inoculated sequentially with PRCV, followed by H1N1-influenza virus 2 days later; the other group was inoculated with H1N1-influenza virus only. Pigs of both groups were serially euthanatized on postinoculation days (PID) 1, 2, 3, and 4 (after influenza virus). At necropsy, influenza virus titer and immunofluorescence in lung tissue were determined and gross lung lesions were recorded.(ABSTRACT TRUNCATED AT 250 WORDS)