The validity of in vivo fluorometric assays was ascertained for phage and bacterial DNA measurements. The following parameters were determined by this simple technique. The DNA content of dividing cells of Bacillus subtilis 168/2 was 2.65 times higher than in resting cells. Assuming that resting cells harbor 1 genomic equivalent, its Mr was estimated to be 4.4 x 10(9) Da. A polymerization rate during growth of 788,000 bp min-1/cell is accounted for by a multifork replication mechanism. Both phage and host DNA could be measured accurately during the lytic cycle. Phage 2C DNA synthesis proceeded at a linear rate of 5.2 genome equivalents min-1.