The effects of the cannabinoids Δ9-tetrahydrocannabinol (THC), cannabinol and cannabidiol on the primary humoral immune response, the secondary humoral immune response and the memory aspect of humoral immunity in response to sheep red blood cell (SRBC) immunization was investigated. Mice treated with THC (10 and 15 mg/kg) during the primary immunization period exhibited a suppression of the primary humoral immune response. Mice treated with THC during the secondary immunization period showed no measurable suppression of the secondary humoral immune response to the immunizing antigen. The memory aspect of humoral immunity was assessed when treatment with cannabinoids was carried out during the primary immunization period and the ability of mice to undergo a secondary immune response was evaluated; suppression of the secondary humoral immune response was evident with THC treatment (10 and 15 mg/kg). Cannabinol and cannabidiol (10 and 25 mg/kg) treated mice showed no impairment in the ability to undergo primary or secondary immune responses with any treatment protocol. In vivo investigations of the effects of cannabinoids on the thymus were also carried out. Thymus weight and thymus cell number were depressed in mice undergoing a primary humoral immune response when treated with THC (10 and 15 mg/kg) during this period. THC treatment, however, did not alter these parameters in mice not challenged with antigen. In both challenged and unchallenged animals, cannabinol and cannabidiol did not measurably alter the thymus.