The administration of actinomycin D to fasted mice 12 hours before killing results in marked disaggregation of hepatic polyribosomes as determined by sucrose density gradients prepared from deoxycholate-treated postmitochondrial supernatants. This disaggregation of hepatic polyribosomes occurs in the free but not in the membrane-bound polyribosomes. Free, but not membrane-bound, polyribosomes of livers of actinomycin D-treated animals demonstrate decreased incorporation in vitro of 14C-leucine into proteins. © 1969.