The efficiency of labeling DNA with Taq DNA polymerase for probing nucleic acid blots was evaluated as an alternative to the more common procedure of using the Klenow fragment. The DNA was labeled with Taq DNA polymerase using random primers. The DNA was labeled specifically and efficiently. Synthesized DNA showed fragments of sizes smaller than those produced by the Klenow fragment and could be performed with as little as 0.5 pg of DNA. The use of Taq DNA polymerase appeared to be limited by the amount of radiolabeled nucleotide used and was more sensitive to non-optimized conditions in the reaction mixture than the Klenow fragment. The relative amounts of incorporated nucleotide in comparable conditions were, on occasions, 10% to 25% lower with Taq DNA polymerase than when using the Klenow fragment; nevertheless, the use of the Taq DNA polymerase to label DNA with random primers offers a very good alternative to the Klenow fragment as shown by this report.