A new, simple, and inexpensive method for the rapid isolation of DNA from whole blood is described. Cell nuclei are prepared by lysis of cytoplasmic membranes and DNA within the nuclear pellet is dispersed with guanidine isothiocyanate and precipitated with isopropanol. DNA prepared in this way restricts completely and results in low backgrounds of nonspecific hybridization after Southern analysis. The yields of DNA are similar to those obtained by more tedious traditional procedures. Numerous genomic DNA samples can be prepared from whole blood in 2 h, thus facilitating gene linkage or other molecular studies in which large numbers of individuals are required.