Using a set of synthetic overlapping peptides, encompassing the entire N-terminal domain (HN,) of the heavy (H) chain of botulinum neurotoxin serotype A (BoNT/A), we have mapped on HN, the regions recognized by Abs (B cells) and by T cells in two inbred mouse strains. After one BoNT/A toxoid injection, BALB/c T cells mounted a weak in vitro response to a region within overlap 687-705/701-719. The remaining peptides stimulated no detectable responses. After 3 injections, BALB/c T cells gave stronger responses to an expanded region within the overlap 687-705/701-719/715-733, peaking at 701-719. BoNT/A-primed BALB/c T cells showed substantial cross-reaction with BoNT/B but did not respond to TeNT. Unlike BALB/c T cells, BoNT/A-primed T cells of SJL cross-reacted well with both BoNT/B and with TeNT. They also recognized a lager epitope profile than the corresponding BALB/c T cells. After one injection with BoNT/A toxoid, SJL T cells responded in vitro to a number of the HN peptides. Regions within peptides 617-635 and 561-579 stimulated strong in vitro responses. Several peptides (463-481, 589-607, 659-677, 729-747, 827-845, and 841-859 revoked weak-to-medium proliferative activities. Four other peptides stimulated very low bu reproducible responses (SI between 2.0 and 3.0). After 3 BoNT/A injections, SJL T cells responded in vitro strongly to peptides 463-481, 561-579, 617-635, 743-761, and 841-859. There were medium or weak responses to at least 10 other peptides. The cells also responded well to the L-chain peptide 218-231. Antisera of BALB/c and SJL obtained after 3 injections with BoNT/A toxoid, protected at very high dilution recipient mice against LD105 of BoNT/A. BALB/c Abs showed medium-to-high binding to peptides 533-551/547-565, 785-803, and 813-831/827-845. Four other peptides showed very low binding. The corresponding SJL Abs had high binding to the overlap 533-551/547-565/561-579, and peptides 743-761, 785-803, and 813-831. Thre other peptides bound low amounts of Abs. The results indicate that the responses teach Ab or T cell epitope is under separate genetic control and that, in a given strain the Ab and T cell recognition regions may coincide but, in addition, HN contains regions that are recognized only by Abs or only by T cells.