During the epizootic spreading in Europe, equine influenza was brought in also into Czechoslovakia. The disease gradually spread from Slovakia (where horses were isolated after a trip abroad) to race horses throughout the country. The serological examination by the IH test showed that, again, the A equi 1/Praha 56 virus was the causative agent of the infection. The application of the Grippeguin vaccine from France was effective. No clinical manifestations of the disease were observed in two mares from one farm that had not been vaccinated although they had been in the focus of infection. However, these mares were housed together with one-year-old foals vaccinated against equine influenza before transport to the training stable in autumn 1972. In addition, the disease was not brought upon their two foals in the focus of infection (the age of these foals was three and five months).