Aminoacyl-tRNA SYNTHETASE COMPLEXES EXISTING IN Chinese hamster ovary (CHO) cells were shown to undergo alterations as a function of the growth state of the cell. The distribution pattern for 13 particulate postribosomal aminoacyl-tRNA synthetases in 10-30% (w/v) exponential sucrose gradients was determined for the enzymes from CHO cells as they exist under three different culture conditions: exponential growth, G1 arrest induced by isoleucine deficiency, and G1 arrest induced by leucine deficiency. The synthetases specific for the amino acids Arg, Asp, Cys, Gln, His, Lys, Met, Thr, and Val have indistinguishable distribution patterns in all three cell types. However, the synthetases specific for Glu, Pro, Leu, and Ile have a unique distribution of synthetase forms in the G1-arrested cultures and this distribution is independent of whether G1 arrest was induced by isoleucine or leucine deficiency. The distribution of synthetase forms in G1-arrested cells differs in a definite, reproducible manner from the profiles obtained with the exponentially growing cells, and this fact is strong evidence for an in vivo role for the synthetase complexes.