The effects of patulin and T-2 toxin were investigated on immunological responses of Balb/c mice. In vitro patulin had a stimulatory effect on splenocytes at lower concentration (1 nM to 10 nM) and strongly inhibited lymphocyte proliferation at higher concentrations (ID50 from 0.02 to 0.24 microM depending on mitogens). In the same experiments T-2 toxin was 100-fold more potent (ID50 from 0.7 to 2 nM). In vivo studies on immunity were performed in mice receiving Bordetella pertussis antigens and keyhole limpet haemocyanin. Patulin significantly reduced delayed type hypersensitivity to B. pertussis antigen and did not reduce anti-KLH antibody production. T-2 toxin had no effect on delayed type hypersensitivity and reduced anti-KLH antibody production. Splenocytes were harvested in mice with or without antigen stimulation to assess mitogenic responses. Patulin generally increased splenocyte proliferation, therefore T-2 toxin effect depended on the immunological status of mice and on the dose injected. At the lower doses (0.8 mg/kg), T-2 toxin enhanced responses to mitogen, but at the greater dose (1.6 mg/kg) T-2 toxin enhanced responses to mitogen of antigen stimulated mice and decreased responses of unstimulated mice.