Juvenile and adult mice (4 and 7 weeks old, respectively) were fed various levels of 2, 3, 7, 8-tetracholorodibenzo-p-dioxin (TCDD) incorporated in mouse feed for five weeks or more. Animal parameters monitored included body weight, organ weights, white blood counts, hematocrits, certain serum protein levels (see below), symptoms of overt toxicity and mortality. High exposure levels (100 ppb) produced marked suppression in total serum protein, gamma globulin and albumin, but an increase in the beta-globulins. Feeding levels of 10 ppb TCDD or more reduced the primary and secondary antibody response to both tetanus toxoid and sheep erythrocytes. The amount of suppression appeared to be dose related, with juvenile animals showing greater suppression than adults. Antibody suppression from the 10 ppb feed level was roughly equivalent to that observed from a single high dose (200 mg/kg) of cyclophosphamide (CY). No evidence of enhanced IgE synthesis was obtained from TCDD exposed animals. TCDD feeding also lowered contact sensitization to dinitrofluorobenzene and resistance to challenge with either Salmonella typhimurium or Listeria monocytogenes.