Procedures for effective cellular lysis and plasmid deoxyribonucleic acid (DNA) isolation from group N streptococci were developed. Cells were grown at 32 degrees C for 4 h in a modified Elliker broth containing 20 mM DL-threonine. After cellular digestion with 2 mg of lysozyme per ml for 7 min at 37 degrees C, 1% sodium dodecyl sulfate exposure resulted in complete and immediate lysis. Lactose (Lac) plasmid species in Streptococcus lactis C2 and S. cremoris B1 (30 and 37 megadaltons, respectively) were demonstrated upon examination of DNA from the cleared lysates by agarose gel electrophoresis. Increasing the lysozyme treatment to 20 min or more resulted in loss of the Lac plasmid, whereas other resident plasmids were unaffected and demonstrable in agarose gels. Diethylpyrocarbonate added before lysis prevented Lac plasmid loss in 20-min lysozyme-treated cells, but was not effective after 40 min of lysozyme treatment. The results suggested that endogenous nuclease activity during the lysozyme treatment period initiated Lac plasmid DNA loss. The development of an efficient lysis procedure for the group N streptococci allowed rapid identification and characterization of plasmid DNA by agarose gel electrophoresis. The plasmid composition of S. lactis C2 and S. cremoris B1, as determined by agarose gel electrophoresis, compared favorably to previous electron microscopic observations.