We examined the level of antigenic conservation amongst the haemagglutinins (HAs) of H1 swine influenza viruses, recently isolated from a wide geographical area, in haemagglutination inhibition assays against a panel of four monoclonal antibodies (MAbs). We found a high degree of conservation with a dominant variant (52 of 54 isolates) that reacted with all MAbs. Only two minor variants, each failing to react with one MAb, were found. Using a one-step PCR technique followed by direct sequencing of the products, we examined the HA1 region of the HA RNA of two representative dominant variants. We found no amino acid substitutions relative to a reference strain. The sequences of the HA1 RNA of the two minor variants isolated here and of two other minor variants defined previously were also determined. Each contained inferred amino acid substitutions, all located at different positions on the HA. Finally, we sequenced HA1 RNA obtained from the original pig lung suspensions from which the two dominant and two minor variants had been isolated. Three of the parent viruses were identical to their progeny in eggs whereas the fourth parent virus contained four amino acid differences from its progeny.