In guinea pigs, the type C and/or type D antitoxin responses to a single dose of a bivalent or monovalent Cl. botulinum vaccine increase markedly between the fourth and ninth week after injection and still increase markedly by the ninth week. For type C, a similar pattern has been found in cattle. Antigens of types C and D mutually interfere with the antitoxin responses in guinea pigs. Graded doses of vaccine arouse graded antitoxin responses in guinea pigs. Stability trials of vaccines have emphasized the unsatisfactory nature of an absolute-response type of assay, rather than revealing any loss of potency during storage. Attention is drawn to the need for a graded-response type of assay in which vaccines under test are compared with a reference vaccine.