Ochratoxin A contamination of cereal feed grain was monitored during October 1989-September 1990 by analysis of blood samples from slaughter swine in Sweden. The detection of ochratoxin A in swine blood was used as a method to identify swine herds fed ochratoxin A contaminated feed. The contamination level of ochratoxin A in the blood of the positive herds was in the range 2-45 ng/ml with the mean concentration 5.2 ng/ml. Feed samples for mycological analysis were collected from both ochratoxin A positive herds (greater than or equal to ng/ml blood) and ochratoxin A negative herds (less than 2 ng/ml blood). From the ochratoxin A positive herds and the ochratoxin A negative herds 22 and 21 feed samples were collected, respectively. No quantitative differences in mould content, as determined by colony forming units, were observed between the two groups. However, there were differences in the mycoflora. The incidence of storage fungi (Penicillium and Aspergillus spp.) was significantly higher (p less than 0.05) in feed from ochratoxin A positive herds. Particularly, Penicillium verrucosum was found to be significantly more common (p less than 0.001). Altogether 274 isolates were screened for their ability to produce ochratoxin A. Ochratoxin A producers were found only within P. verrucosum; 38% of the 63 isolates produced detectable amounts of ochratoxin A. Ochratoxin A producing isolates of P. verrucosum were found in 60% of the feed samples collected from ochratoxin A positive swine herds and in one sample (5%) of the feed samples collected from the ochratoxin A negative herds.