A new system is described for studying the control of protein synthesis. In a monolayer culture of chick embryo liver cells, plasma proteins are synthesized for three days at in vivo rates. The plasma proteins are secreted into the culture medium and without concentration are detected there simply and sensitively by a modified Laurell electronimmunoassay. Secretion of the newly synthesized plasma proteins occurs within 30 min of their synthesis. Thus, rates of synthesis of the plasma proteins can be followed readily from rates of their accumulation in the culture medium. This system has the following advantages for the study of protein synthesis: cells do not have to be disrupted for the assay; the cell population can be followed over several days; it is not necessary to label the proteins radioactively; and turnover of plasma proteins is negligible and need not be taken into account. The usefulness of the system is illustrated by a number of findings. The spectrum of plasma proteins synthesized in culture changed qualitatively and quantitatively. Albumin synthesis steadily decreased with culture time and stopped at the third day, whereas the synthesis of some new plasma proteins ("adult") was induced. These qualitative changes suggest differential gene expression in culture and a special control of albumin synthesis in vivo, different from the synthesis of the other plasma proteins. Quantitative changes in the rates of synthesis of specific plasma proteins suggest a competition among their messenger RNAs for components of the translational machinery. Insulin has a differential effect on the synthesis of specific plasma proteins at concentrations within the physiological range of the hormone.