The isolation of DNA from whole blood by a modified rapid method (RM) was tested using various detergents and buffer conditions. Extraction of DNA with either NP-40 or Triton X-100 gave a high yield of undegraded DNA in less than an hour. The concentration of magnesium ion in the buffers was critical to obtaining intact, high molecular weight (HMW) DNA. Greater than 10 mM MgCl2 led to degradation. Addition of EDTA to the buffer inhibits this degradation. Preparation of DNA from blood stored at room temperature or incubated at 37 degrees C for 24 hr resulted in the same amount and quality of DNA as from samples frozen at -70 degrees C. DNA from blood samples that had undergone more than four freeze-thaw cycles was found to be partially degraded. The modified RM can be applied to extract DNA from as little as 10 microliters of blood (340 ng of DNA) and from dried blood samples. DNA samples remained intact and undegraded for longer times when DNA was dissolved in higher concentrations of EDTA.