Globin mRNA levels in 11–15-day mouse fetal liver cells have been estimated by in situ hybridization of a highly labeled DNA copy (cDNA) of adult globin messenger RNAs (mRNAs) (globin cDNA) to fixed preparations of cells. Under the conditions employed, no significant in situ hybridization occurred to lymphoma cells (L 51787), mouse L cells, or hepatocytes; whereas reticulocytes from phenyl hydrazine-treated mice showed extensive in situ hybridization. The proportion of fetal liver cells showing predominantly cytoplasmic in situ hybridization increased from about 30% at the 11th day of development to 80–85% by days 13–15. Unlike more mature cells, proerythroblasts did not show in situ hybridization, except to a slight extent at later stages of development. These studies therefore indicate that globin mRNAs begin to accumulate during or shortly after the proerythroblastbasophilic erythroblast transition. The fact that certain immature erythroid cells from 14-day fetal liver contain substantial amounts of globin mRNAs has been confirmed by comparing the hybridization in solution of globin cDNA to cytoplasmic RNA extracted from total fetal liver cells or from immature erythroid cells obtained by treatment of fetal liver cells with an antiserum raised against erythrocytes
