Infant botulism is an infectious form of a disease heretofore principally known as food-borne intoxication. Previous epidemiologic and laboratory studies have shown that infant botulism results from the ingestion of spores of Clostridium botulinum that subsequently germinate in the infant intestine and produce botulinal toxin. A quantitative study of the fecal microflora of four infants with infant botulism revealed the presence of C. botulinum in numbers as high as 6.0 x 10(8) colony-forming units (cfu)/g. At various times after the onset of illness, the numbers of C. botulinum that were recovered from feces ranged from 10(3) to 10(8) cfu/g and constituted from 0.01% to 3.3% of the total fecal flora. It was concluded that the large numbers of C. botulinum found in patients' feces could occur only as a consequence of in vivo spore germination and outgrowth.