Twenty of 80 strains of Penicillium roqueforti were able to produce up to 600 mg of mycophenolic acid (MPA) liter-' in 2% yeast extract-5% sucrose broth. Sixtytwo of these strains had been isolated from the main blue-veined cheese varieties of western Europe or from starter cultures. Of these 62 dairy strains, only 7 had MPA-producing potential in vitro. These seven strains had all been isolated during the period 1975 to 1981 from the blue cheese of one individual factory. In cheese from the market, MPA (up to 5 mg kg-1) was only found in samples of this same factory. With MPA-producing and-nonproducing strains for the experimental manufacture of blue cheese, MPA synthesis in cheese was only detected with strains which form MPA in yeast extract-sucrose broth. The maximum MPA level at 4 mg kg-1 was similar to that in commercial cheese. Toxicity of MPA was tested with two established human cell lines (Detroit 98 and Girardi Heart) and one established pig kidney cell line (AmII).