Patulin adduct was formed by reacting equimolar amounts of patulin and cysteine at pH 6.0. The bactericidal effect of the adduct mixture was less than one hundredth of that of free patulin in plate and liquid cultivation tests with Escherichia coli W 3110 thy pol A1 and pol A1+ strains. The bacterial strains did not liberate free patulin from the adduct mixture present in the growth medium. Neither patulin nor the adduct mixture induced repair effects in E. coli. The oral LD50 of patulin for NMRI male mice was 29 mg/kg. For the adduct mixture, not even as high a dose as 2,370 mg patulin equivalents per kg body weight caused any deaths or macroscopic pathological alteration. Neither patulin nor the adduct mixture was mutagenic to the histidine auxotroph Salmonella typhimurium strains TA 100 and TA 98 in microsomal activation tests or to TA 1950 and 1951 in host-mediated assay.