Replication of DNA in mammalian cells in S phase is initiated at about 50,000-100,000 sites (Hand & Tamm 1974, Jasny eta]. 1978, Cohen et al. 1978). In general, replication proceeds bidirectionally from the origins (Huberman & Riggs 1968, Edenberg & Huberman 1975). There appear to be many more potential initiation sites than those activated during S phase. Evidence for this comes from work in developing organisms (Callan 1972, 1974) and with 5fluoro-2'-deoxyuridine (Taylor & Hozier 1976, Taylor 1977). We have recently carried out a detailed examination of DNA chain initiation by light microscopic DNA fiber autoradiography in muntjac, mouse (L-929), and bovine (MDBK) cells in culture (Jasny et al. 1978, Cohen et al. 1978). Exponentially growing cells were labeled with high specific activity 3~-thymidine for 10 minutes and then with low specific activity 3~-thymidine for 3 hours. The modal inter-initiation site distance is 5-15 pm (15,00045,000 base pairs) in all three cell types, but the individual measurable distances between the activated sites vary from 3.4 to 432 pm. Statistical tests of randomness, applied to the distribution of inter-initiation distances, show a nonrandom frequency distribution of the distances between activated initiation sites for all three cell types.