In studies of ferritin induction, liver slices failed to respond favorably to iron added directly to the suspending medium unless serum was added. The addition of 5 µg Fe/ml to slices in whole serum accelerated the incorporation of radioleucine into ferritin 2.5-fold. The favorable effect of serum on iron-induced ferritin synthesis was correlated with its attenuation of the toxic effects of the iron on protein synthesis; the serum was also found to diminish the uptake of iron by the liver cells. Transferrin, purified from the serum by ion exchange, failed to mediate the iron induction of apoferritin while albumin, completely freed from transferrin as monitored with radioiron, was effective. Albumin, apparently by limiting the uptake of excess iron by the liver cells, modulates the nonspecific toxic effects of iron on protein synthesis and thereby permits the specific induction by iron of the biosynthesis of apoferritin.