DNase I production in transgenic mice. (a) DNase secretion from lymph node cells was quantified by DNA-methyl green (DNA-MG) assay. Cells were placed in culture for 3 days either in (a) the absence or (b) presence of anti-CD3 antibodies. Supernatants were diluted in assay buffer 1:2 for unstimulated cells and 1:16 for stimulated cells. (c) Activity of the DNase I present in the supernatants purified as in (b). The activity was determined in the presence of the normal DNA-MG assay buffer, described in Materials and methods, or following the addition of 0.9% NaCl/ATP or NaCl/ATP/Actin. The level of DNase I activity in the supernatants was adjusted to give similar levels in the normal DNA-MG assay buffer. (d) DNase I activity in the urine of actin-resistant, salt-resistant and hyperactive mutant of DNase I (ash.DNase I) transgenic mice compared to control C57BL/6 littermates. wt.DNase I, wild-type DNase I.