Abstract
DNA''s from lambda phages of different genotypes sediment at increasing rates in the order b2b5, b2, b5, b+, which is the same as the order of increasing densities of the phage particles. DNA from b+ phage is more fragile than that from b2b5. The two mutations studied are therefore accompanied by characteristic reductions in the lengths of DNA molecules, reductions that are additive in the recombin-ant phage carrying both mutations.