A panel of H-2k class II-restricted Th clones were established from individual CBA mice primed by infection with X31 influenza virus. 27 clones, which showed specific recognition of the HA surface glycoprotein, were all H3N2 subtype specific, in contrast to a T cell line which was crossreactive and which may have other specificities. 20 distinct HA-specific clones recognized a tryptic cleavage fragment of X31 consisting of residue 28-328 of HA1 (tops) which includes all the Ab-combining regions of the HA molecule. Seven other HA-specific clones failed to respond to either tops or to aggregate (the remainder of the virus after tryptic cleavage of tops). The specificity of these clones has been mapped, tentatively, to a conformational determinant in the interface antibody-binding region of the HA trimer. Analysis of the fine specificity of the HA-specific clones against a panel of H3N2 natural variant viruses isolated from major virus epidemics from 1968 to 1984 revealed a hitherto unrecognized diversity in T cell recognition of a HA. A total of 12 specificity groupings were evident, and varied from groups of clones that recognized all natural variants to one clone that responded only to isolates from 1968 to 1972. Six out of eight clones from a major specificity group, which failed to recognize variants TX/77, BK/79, or CN/84, responded to two overlapping peptides (48-68 and 53-87), corresponding to a region of HA1 that includes part of two antibody combining sites. An examination of the amino acid sequences of natural variant viruses from this region of HA revealed that residues Asn53 and Asn54 and/or Ile62 were critical for recognition by these clones. We conclude that recognition of HA by Th cells is not restricted to a limited number of epitopes in the conserved regions of the molecule, but is extremely diverse and includes specificities that map to variable antibody-combining regions of the molecule. In addition, the sensitivity of the T cell clones to the amino acid substitutions occurring in HA1 of natural variant viruses suggests that Th may play a role in the immune pressure for antigenic variation in the HA molecule.