The effects of T-2 toxin, diacetoxyscirpenol, ochratoxin A and zearalenone on DNA synthesis in phytohaemagglutinin-stimulated human peripheral blood lymphocytes were studied by assaying the incorporation of [3H]thymidine. Total inhibition was obtained with 8 ng T-2 toxin/ml, 8 ng diacetoxyscirpenol/ml or 30 micrograms zearalenone/ml, and with 20 micrograms ochratoxin A/ml inhibition was almost complete; 50% inhibition was produced by 1.5 ng T-2 toxin/ml, 2.7 ng diacetoxyscirpenol/ml, 14 micrograms zearalenone/ml or 14 micrograms ochratoxin A/ml. The toxicity of the trichothecenes to the lymphocytes was slightly reduced when rat liver cells were present whereas the toxicity of ochratoxin A and zearalenone was unaltered. Low concentrations of trichothecenes did not produce any inhibition of DNA synthesis when the cultures were washed and placed in fresh media containing only phytohaemagglutinin 4 hr after the addition of the test compounds. Sister chromatid exchange frequency was not elevated by T-2 toxin, diacetoxyscirpenol or ochratoxin A. Zearalenone had a weak enhancing effect on the frequency of sister chromatid exchanges.