Clostridium botulinum; Wikimedia Commons
Although the United States did conduct some preliminary tests with the highly toxic compound ricin at the end of WWI, the development of biological weapons, felt to be impractical, was not emphasized in the inter-war period.
As late as 1942, the USA had no biological weapons capabilities. However, by November 1943 a biological weapons facility was opened at Fort Detrick, Maryland along with three other facilities in Indiana, Mississippi and Utah.
The U.S. biological weapons program focused primarily on the offensive potential of botulinum toxin, anthrax and smallpox. Unlike Japanese and Nazi programs, no human experimentation was performed.