Seed in the NPGS base collection comes from USDA-ARS-NPGS regional or crop specific field sites.  At the field sites, seeds are dried to ambient conditions and cleaned to remove empty seeds and chaff.  A sample of seed from each accession is retained at the field site for regeneration, multiplication, distribution, characterization and evaluation and comprises an active collection.  The NPGS base collection contains all the inventories of each accession including the original sample and earlier regenerations. This consolidated collection of all of NPGS holdings is stored at the NCGRP in secure freezers at -18°C.  The viability of stored seeds is periodically monitored using standard germination assays.   A fresh sample of seed is obtained if seed supply is too low or germination percent decreases below about 60%.  A fresh inventory usually contains between 1500 and 3000 seeds. 

As of July 1, 2008 the NPGS base collection stored at the NCGRP consisted of:

Number of samples	505,770
Number of unique accessions	384,876
Number of genera	1,180
Number of species	6,968
Number of seed accessions	380,727
Number of vegetatively-propagated accessions	4,177
Percent of NPGS accessions backed-up	76%
Percent back-up of vegetatively-propagated accessions	8%