One blue-type cheese starter and 11 isolates of P. roqueforti from different foodstuffs were tested for the production of PR-toxin. 3 strains were able to produce the toxin on rice at 15 degrees C and sometimes at 30 degrees C. The toxin quantities detected were 310-960 ppb. The starter culture didn't produce any PR-toxin. The investigations with P. roqueforti NRRL 849 showed a toxin-production in the temperature range of 15-30 degrees C, with an optimum at 15 degrees C, whereas 25 degrees C was the optimum for growth. PR-toxin was determined by thin-layer-chromatography and in doubtful cases by mass-spectrometry.