ABSTRACrNucleoli from auxin-treated tissues (Glycine max L. var Wayne or Kaoshiung No. 3) were isolated and purified by Percoll density gradient centrifugation. There was a 2.1-fold increase in RNA and a 2.8fold increase in protein after a 24-h auxin treatment per unit nucleolar DNA. More than 150 acid-soluble protein spots were associated with the auxintreated nucleoli on two dimensional (2-D) gel electropherograms.Nucleoli from auxin-treated tissue were fractionated by suspension in 20 millimolar dithiothreitol at room temperature for 20 minutes into two distinct fractions referred to as the nucleolar chromatin and preribosomal particle fractions. The DNA:RNA:protein ratio ofthe chromatin fraction was 1:2.5:14. Most of RNA polymerase 1 activity and nucleolar DNA recovered in this fraction. The acid-soluble proteins in the chromatin were resolved into 32 protein spots on 2-D gel electropherogram. The most abundant spots were identified as histones.The nucleolar preribosomal particle fraction had a DNA:RNA:protein ratio of 1:24:102 and contained only trace amounts of RNA polymerase 1 activity and only 10 per cent of the nucleolar DNA. Acid-soluble proteins associated with these particles were resolved into 78 protein spots; 72 of these (acid-soluble) protein spots corresponded in 2-D gel electrophoresis to 80S cytoplasmic ribosomal proteins. Some 15 protein spots found in 80S ribosomal proteins were absent in the preribosomal particles. It seems reasonable, based on these data, that the enlargement of nucleoli after auxin treatment is primarily due to the large increase in ribosomal proteins and rRNA which accumulate and assemble in the nucleoli in the form of preribosomal particles.A substantial body of literature has established that the synthesis and processing of pre-rRNA occurs in the nucleolus of eukaryotic organisms (2,22,29,31). The nucleolar organizer region (15) contains several hundred to several thousand copies of rRNA genes which are associated with proteins in the form of nucleolar chromatin. During the transcription of pre-rRNA, several ribosomal proteins become associated with the nascent RNA; this leads to further processing and packaging of the prerRNA in the nucleolus to form ribonucleoprotein complexes termed preribosomal particles or preribosomes (29).The nucleolar volume as a fraction of total nuclear volume varies considerably as tissues undergo altered rates of ribosome production. For example, partial hepatectomy or injection of thioacetamide into rats results in an increase in relative nucleolar