The aim of the study was to investigate the infection dynamic of Haemophilus (H.)parasuis and Mycoplasma (M.) hyorhinis in 3 farms. A total of 61 piglets were clinically investigated at 1., 3., 5., 7., 9., 11., 14., 18. and 22. weeks of life and a blood sample was taken from each piglet as well as from the sows. The serum samples were tested using ELISA for antibodies against H. parasuis and M. hyorhinis. Clinical signs indicating polyserositis were seen in farm 1 and 3. For both pathogens, a decline of the maternal antibodies could be detected up to the 5th or 7th week of life. The duration of persistence depended on the level of the maternal antibodies. In farm 1, all animals were tested positive for antibodies against H. parasuis during the fattening period. In farm 3, several sows were tested positive in the M. hyorhinis ELISA, therefore, positive results in sows can indicate a higher infection dynamic during the fattening period. For H. parasuis as well as for M. hyorhinis a significant correlation between the level of the antibodies in the sows and their piglets could be seen.