Interferometric and photometric measurements have been made on replicating embryo human cell cultures. From a study of the relations between successive physical measurements on individual cells, it was found that the net syntheses of DNA, nuclear RNA, nuclear protein, and cytoplasmic RNA are closely associated during interphase. Additional measurements of DNA and cytoplasmic RNA on freshly prepared replicating monkey kidney cells gave similar results. In auxiliary experiments with embryo human cells, an inhibition of the onset of DNA synthesis (produced by a dose of X-rays) was found to block the majority of the accumulation of nuclear protein and RNA and about half the accumulation of cytoplasmic RNA. These results are consistent with others previously reported in dividing cell cultures freshly prepared from normal tissues.