The effectiveness of ammonia in inactivating aflatoxins in contaminated cottonseed was investigated. Two aflatoxin-contaminated cottonseed lots were treated separately using an atmospheric pressure, ambient temperature ammoniation procedure (APAT) or a high pressure, high temperature ammoniation procedure (HPHT), and incorporated into dairy cow rations. Isocalorific diets containing 25% defatted, dried milk from cows fed aflatoxin-contaminated cottonseed without or with APAT or HPHT treatment, or an aflatoxin-free human grade commercial milk powder, were then fed for 12 months to rainbow trout (Oncorhynchus mykiss). Aflatoxin M1 (AFM1) concentrations in milk powders without and with seed treatment were: APAT, 85 and < 0.05 microgram/kg; HPHT, 32 and < 0.05 microgram/kg. In the APAT experiment, trout consuming the diet containing milk from cows fed the aflatoxin-contaminated cottonseed had a 42% incidence of hepatic tumours; APAT cottonseed treatment reduced this to 2.5%. Positive controls were included to demonstrate trout responsiveness. AFB1 fed continuously for 12 months at 4 micrograms/kg resulted in a 34% tumour incidence, whereas positive controls fed 20 micrograms AFB1/kg, 80 micrograms AFM1/kg, or 800 micrograms AFM1/kg for 2 wk and killed 9 months later had a 37, 5.7 and 50% incidence of tumours, respectively. These data demonstrate that APAT ammonia treatment of aflatoxin-contaminated dairy cattle cottonseed feedstock abolished the detectable transfer of AFM1 or AFB1 into milk powder, and greatly reduced the carcinogenic risk posed by any carry-over of aflatoxins or their derivatives into milk. In addition, the results confirm AFM1 to be a lower level hepatocarcinogen in comparison with AFB1 in the trout carcinogenicity assay. In the separate HPHT experiment, no tumours were observed in the livers of trout fed diets containing milk from either the ammonia-treated or untreated source, or the control diet containing 8 micrograms AFM1/kg. Positive controls fed 64 micrograms AFB1/kg for 2 wk exhibited a 29% tumour incidence 12 months later. Thus in this experiment, neither AFM1 at 8 micrograms/kg nor any HPHT-derived aflatoxin derivatives that might have been carried over into milk, represented a detectably carcinogenic hazard to trout.