The induction of arabinases in Aspergillus niger N400 was studied on different simple and complex carbon sources. Sugar beet pulp was found to be an inducer of three arabinan degrading enzymes (alpha-L-arabinofuranosidase A, alpha-L-arabinofuranosidase B and endoarabinase). These enzymes were purified from A. niger culture fluid after growth of the fungus in medium employing sugar beet pulp as the carbon source and were characterised both physico-chemically (Mw 83,000, 67,000, 43,000 Da and, pI 3.3, 3.5 and 3.0 for alpha-L-arabinofuranosidases A and B and endo-arabinase, respectively) and kinetically (Km on p-nitrophenyl-alpha-L-arabinofuranoside 0.68 and 0.52 mM for alpha-L-arabinofuranosidases A and B, resp.; Km on sugar beet arabinan 0.24 and 3.7 g/l for alpha-L-arabinofuranosidase B and endoarabinase, resp.). The amino acid compositions of the three enzymes were determined also. The enzymic properties were compared with those of arabinases purified from a commercial A. niger enzyme preparation. Differences were found though the kinetic data suggest considerable similarity between the enzymes from the different sources. Antibodies raised in mice against the three enzymes were found to be highly specific and no crossreactivity with other proteins present in culture filtrates was observed. A mixture of these antibodies has been used to analyze specific induction of these individual enzymes on simple and complex substrates by Western blotting.