Dot and slot blotting are simple techniques for immobilizing bulk unfractionated DNA on a nitrocellulose or nylon membrane. Hybridization analysis can then be carried out to determine the relative abundance of target sequences in the blotted DNA preparations. Dot and slot blots differ only in the geometry of the blot, a series of spots giving a hybridization pattern that is amenable to analysis by densitometric scanning. Samples are usually applied to the membrane using a manifold attached to a suction device. The Basic Protocol describes such a procedure for dot or slot blotting on an uncharged nylon membrane; annotations to the steps detail the minor modifications that are needed if blotting onto nitrocellulose. The first alternate protocol describes the more major changes required for blotting with a positively charged nylon membrane. A second alternate protocol describes preparation of dot blots by spotting the samples onto the membrane by hand.