An extraction procedure for the simultaneous isolation of RNA and DNA from tissue culture cells is described. The procedure is a variation of the guanidium/lithium chloride method for RNA isolation which is rapid, simple, and avoids costly ultracentrifugation equipment. The genomic DNA yielded by this procedure is greater than 50 kb in length and may be readily cleaved by restriction endonucleases. Sufficient DNA for Southern blot analysis, and RNA for Northern blot or nuclease protection analysis, can be obtained from as few as 2 x 10(6) cells, making this method particularly suitable for the genetic screening of large numbers of individual, stably transfected cell clones.