A mass spectral database was built using standard instrument software from 678 electrospray mass spectra (mass profiles) from crude fungal extracts of terverticillate taxa within the genus Penicillium. The match factors calculated from searching all the mass profiles stored in the database were used as a distance measure for grouping into taxa. Isolates from more than 75% of the taxa were placed correctly, that is with the highest match factors to other isolates of the their own species by library searches. The database was used as an expert system based on correct identification of the isolates stored according to classical taxonomic criteria. Mass profiles collected in previous studies could be identified by a search in the database. (C) 1997 Elsevier Science Ltd.