Experiments have been conducted that deal with the structure and biological activity of clostridial toxins. Studies have dealt mainly with botulinum neurotoxin, but work has also been done with tetanus toxin and with the binary toxin. Structural studies indicate that proteolytic processing of botulinum neurotoxin induces two major outcomes: activation and aging. The first is associated with a marked increase in toxicity and with conversion from a single chain to a dichain structure. The second is associated with nominal changes in toxicity and with molecular rearrangements in the dichain structure. Immunological studies have resulted in isolation and characterization of a monoclonal antibody that neutralizes tetanus toxin. Monoclonal antibodies have also been raised against botulinum neurotoxin, and these antibodies have been used to demonstrate that: i) activation is not due to marked conformational changes in the relevant epitopes, ii) binding of the toxin to cholinergic nerve endings does not produce detectable conformational changes, and iii) all functional domains of the toxin appear to be internalized simultaneously. Immunological studies done in vivo and in vitro suggest that certain antibodies may enter cholinergic nerves and neutralize subsequently internalized toxin. Additional work on clostridial toxins has produced the following results: i) the ligand binding assay typically used with tetanus toxin (i.e., low pH and ionic strength) is of questionable biological significance, ii) the binary toxin, like the clostridial neurotoxins, enters cells by receptor-mediated endocytosis, and iii) tetanus toxin can alter the disposition of protein kinase C in one neuroblastoma cell line.