The patterns of late replication were compared on direct chromosome preparations from cells of chick embryo lung and kidney, as well as in cultures established from these tissues using tritium autoradiography. Fifty cells from each tissue were examined from each of five animals in vivo and three in vitro. Two- to four-fold differences in the frequencies with which a chromosome arm labeled were seen when lung and kidney were compared; however, labeling differences were often equally significant in the opposite direction when cells from another chick embryo were analyzed. The results obtained in vivo were comparable to those seen in vitro, indicating that cell-culture did not alter cell differentiation in a way that could be detected by late replication patterns. Variation in tissue-specific differences in late replication between individuals suggests that these patterns do not reflect differences in the differentiated state.