Amido black staining of control plates, for comparison of 100 vs. 500 TCID
and 3, 4 and 5 days of incubation. A: Back titrations of both virus concentrations, shown in the upper 3 plates, were carried out on one plate per incubation period, in 4 tenfold dilutions starting from 100 (100 TCID50, upper 4 rows) or 10-1 (500 TCID50, lower 4 rows). B: Results of the control panel of 6 positive (P) and 2 negative (N) samples, are shown in the middle row of 3 plates (100 TCID50) and the lower row of 3 plates (500 TCID50). Serum dilutions are shown above the figures. Results of back titration and average titres are given in Table 1.