Two influenza A viruses were isolated from pigs in Thailand in January 1988 during the early febrile stage of an influenza-like illness. The isolates contained hemagglutinin and neuraminidase antigens related to those of swine H1N1 influenza virus. This result based on the virus isolation is compatible with the epizootiological evidence that, unlike the human influenza with peak activity in summer (May-July), swine influenza virus is prevalent in the winter season (November-January) in Thailand. The proportion of sera with hemagglutination-inhibiting antibody was higher to A/NJ/8/76 than to A/sw/Iowa/15/30. Likewise, hemagglutination-inhibition tests with monoclonal antibodies indicated that hemagglutinin antigen of the isolates was very similar to that of A/NJ/8/76 virus. In agreement with the serological survey and antigenic characteristic, genetic relatedness between the isolates from Thailand and A/NJ/8/76 virus was also demonstrated by the oligonucleotide mapping of RNA, suggesting that they may be of the same origin.