Long-term storage of DNA is required for a number of genetic studies; prior to extraction, blood samples may be subject to elevated temperatures for variable intervals. We have studied the effect of temperatures ranging from -70 degrees C to +65 degrees C on human blood and on DNA extracted from it. DNA in solution stored at ambient temperatures up to 37 degrees C for 6 months was digestible by three different restriction endonucleases, whereas storage at 45 degrees C is deleterious after 6-7 weeks. DNA can be extracted from blood samples stored at -70 degrees C for at least 2 months or at 23 degrees C for a week or more, but blood stored at these temperatures may yield less high-molecular-weight DNA. Cell pellets from which plasma has been removed also can serve as a source of DNA. Isolated DNA stored dry for years (up to 30) is difficult to dissolve and may appear degraded, but a sample stored dry for 13 years and then in solution at -20 degrees C for 7 years appeared to be intact.

The tonB gene product is necessary for the energy‐dependent transport of ferric chelates and vitamin B12 across the Escherichia coli outer membrane. When carried on multicopy plasmids, the cloned tonB gene complemented tonB hosts, restoring transport of ferri‐siderophone complexes and vitamin B12, and susceptibility to the group B colicins and phage ф80. The levels of these activities were all markedly lower than when the tonB+ gene was present in single copy. This depression of TonB function occurred even when the chromosome carried the normal tonB+ allele, but plasmids carrying only a portion of the tonB gene, including the 5′‐regulatory region, were not inhibitory.