A variety of proteins were purified from cotton seeds and were analysed by amino acid sequence determination (automated Edman degradation) and by state-of-the-art electrospray ionization mass spectrometry (ESMS). The amino acid sequences of about 40 vicilin-related cotton proteins were precisely defined. The molecular masses of the deduced sequences were in agreement with the masses observed by ESMS to within about 1-2 Da (i.e. to within the mass of 1-2 hydrogen atoms). A further major cotton seed protein was purified and shown to be a y-conglutin-related protein with 2 component subunits and a molecular mass of 46250.3 1.3 Da. The vicilin fractions and the yconglutin variously have anti-fungal activity against a wide range of fungi tested including the cotton pathogens Fusarium oxysporum and Verticilium dahliae. This precise work is potentially useful for classical and transgenic approaches to cotton plant breeding for pest resistance. Cotton leaves also contain anti-fungal components and a defensive component with larvicidal activity against the blowfly Lucilia cuprina and the mosquito Aedes camptorhynchus. Large-scale purifcation procedures were developed to enable further biological testing. Protein and non-protein fractions from cotton are variously active as inhibitors of animal cyclic AMP-dependent protein kinase and of the proteases trypsin and chymotrypsin