TABLE 3.

Frequency of spectinomycin-resistant mutants in strain B31-A

Spectinomycin concn (μg ml−1)	No. of cells plated	No. of colonies	Frequency (10−6)
1.5	108	572 ± 83a	5.7
	107	69 ± 22a	6.9
6.0	108	694 ± 126a	6.9
	107	62 ± 21a	6.2
30	108	514 ± 118b	5.1
	107	62 ± 5a	6.2

^aValues are means ± standard errors of the means for four independent experiments.

^bValue is the mean ± standard error of the mean for seven independent experiments.