Table 3.

Effect of MTX on focus formation in 1° assay of SA′ cells and subsequent 2° assay of the cells without MTX

Assay	MTX (nM)	Series A	Series B
		% of focus formers
1°	0	0.013	0.010
	200	<0.001	<0.001
2°	0 (0)*	1.05	0.75
	0 (2000)*	0.086	0.023

Two parallel low-density passages of SA′ cells were seeded in MCDB 402 with 2% CS for a 1° assay. MTX (200 nM) was added to the medium of half the cultures from days 3 through 6. Treated and untreated cultures were fixed and stained at 14 days. Sister cultures were used for a 2° assay without MTX. The percentages of foci in each assay are recorded. 

^*The value (in parentheses) for MTX concentration in the 2° assay refers to the concentration applied from days 3–6 of the 1° assay. MTX was not used in the 2° assay.