The distribution of 75Se selenite in various macromolecules over time was evaluated in confluent cultures of the LMA mouse mammary epithelial cell line. 75Se label did not distribute equally to all fractions at the same time point. The label initially entered the selenonucleic acid fraction and subsequently increasing quantities were detected in the selenoprotein fraction. The time period of increasing inhibition of DNA synthesis correlated with increasing levels of selenoproteins. The major selenoprotein found in this time frame in confluent phase LMA cells was an acidic, 58K selenoprotein.