Despite the importance of avian P450 forms in modulation of the toxicity of pesticides and other environmental chemicals, relatively little work has been done upon them, and very few forms have been fully characterised. An avian form that appears to belong to family 1A is readily inducible by planar molecules (e.g. coplanar PCB's, PCDD and certain PAHs) and has been the basis of a biomarker assay used in field studies. Although it is recognised by antibodies for mammalian P450 1A1, it evidently differs from the mammalian forms of the enzyme in catalytic properties. Phenobarbitone induces two forms of P450 in the domestic fowl (2H1 and 2H2) which have been purified, and these resemble P450 2B1 and P450 2B2 of the rat respectively. Two further phenobarbitone inducible forms, PB-A and PB-B have been partially purified. Also there is an acetone inducible form that resembles rodent P450 2E. In field studies evidence has been produced for the induction of P450s recognised by antibodies to mammalian forms of P450 1A1 and P450 2B in avian liver (adults and embryos), in response to environmental levels of PCBs. Fungicides which act as ergosterol biosynthesis inhibitors (EBI fungicides) such as prochloraz and propiconazole potentiate the toxicity of certain phosphorothionates to birds.