Nine aminoacyl-tRNA synthetases from Ehrlich ascites cells were examined with respect to their ability to be isolated as high molecular weight complexes, soluble enzymes, and ribosome-bound enzymes. Several different methods were employed for cell homogenization and enzyme isolation, with particular attention paid to the effects of hypotonic, isotonic, and hypertonic buffers on enzyme isolation. The binding of all synthetases to ribosomes was eliminated if the low ionic strength of the isolation buffer was raised to isotonic levels. In contrast, neither the ionic strength or composition of the buffers, nor the procedures used for cell homogenization or enzyme isolation had any significant effect on the isolation of the high molecular weight synthetase complex. Certain enzymes (lysyl-, methionyl- and isoleucyl-tRNA synthetases) formed very stable complexes and high molecular weight species were the predominant forms of these enzymes under all conditions of cell homogenization and enzyme isolation. Other enzymes (glycyl-, tyrosinyl- and threonyl-tRNA synthetases) formed complexes very weakly, if at all, and always appeared predominately in the soluble enzyme fraction. Isolated soluble forms of the lysyl-, methionyl- and isoleucyl-tRNA synthetases did not associate to form significant amounts of complex upon re-isolATION, SUGGESTING THAT A COMPONENT NECESSARY FOR COMPLEX FORMATION WAS MISSING FROM THE SOLUBLE ENZYME FRACTION. However, the soluble forms of these enzymes, but not the glycyl-, tyrosinyl- and threonyl-tRNA synthetases, did for complexes when mixed with ribosomal RNA or polyuridylic acid. Preliminary experiments showed no significant differences between the complexed and soluble forms of the lysyl-, methionyl- and isoleucyl-tRNA synthetases with respect to Km values or ability to charge different isoaccepting tRNAs.