Escherichia coli grown in medium containing heavy isotopes were treated with chloramphenicol in a medium containing heavy isotopes and [ 3H]uridine. The fate of the radioactive heavy CM §-particles formed during this time was followed in non-radioactive light medium in the absence of CM. Radioactive RNA contained in CM-particles was converted to RNA in normal ribosomes. Density-gradient analysis of the resultant radioactive ribosomes showed two major peaks. The lighter radioactive ribosomes may have derived their radioactivity from breakdown products of the pre-existing RNA. Thus, radioactive precursors mix with newly synthesized light precursor materials and are repolymerized. The presence of a major component of heavier radioactive ribosomes suggested that at least a part of the RNA in CM-particles was converted without breakdown to RNA in normal ribosomes. As a control, experiments were performed to examine the conversion to ribosomes of pulse-labeled RNA synthesized under conditions identical to those of the previous experiment. The density-gradient analysis of the radioactive 50 s ribosomes derived from pulse-labeled RNA showed that most of the RNA in the 50 s ribosome was synthesized from pulse-labeled RNA after an extensive mixing of the label with light precursor materials. An analysis of ribosomes at an earlier time after the transfer to the light medium showed that this conversion takes place by a mixing of labeled precursors with the light precursors simultaneously with the formation of ribosomes from intact CM-particles. Conversion to RNA of ribosomes of pulse-labeled RNA synthesized in the absence of CM was also studied using the same experimental approach. Conversion to RNA of 50 s ribosomes was found to involve an extensive mixing of the label with the newly created pool materials. Only a very small amount of high molecular weight precursor RNA was detected among pulse RNA in this case. The present experiments failed to provide a definite conclusion as to how pulse-labeled RNA is converted to RNA of the 30 s ribosome.