Effect of sonic extract from strain UP54 on the survival of HEp-2 cells. CDT-containing extract (0.06, 0.6 and 6.0 of total protein) was added to cells, in triplicate wells, on days 1, 2, 3, 4 and 5 of growth. All plates were incubated for a total of 8 days, after which the cells were fixed and stained and the colonies counted by computer. Data were plotted as described in the legend to Fig. 2.