Poliovirus types 1, 2 and 3 (Sabin strains) were titrated using microtissue culture plates in three different cell substrates: Hep 2C, Vero and MRC-5 human diploid cells. Each type of poliovirus was titrated 60 times. The virus dilutions were inoculated in the three cell substrates and the titres (TCID50/ml) calculated by the Spearman-Kärber method. The results were analyzed by the U-test (Mann and Whitney procedure) in a Wang 600 computer. For poliovirus type 1 the differences in titres observed between Hep 2C and Vero cells were not statistically significant at p = 0.01, but the differences in titres between Hep 2C and MRC-5 human diploid cells and between Vero and MRC-5 cells were statistically highly significant (p = 0.001). For poliovirus types 2 and 3 the observed differences in titres were highly significant (p = 0.001) between Hep 2C and Vero cells, Hep 2C and MRC-5 and also between Vero and MRC-5 cells, with higher titres on Hep 2C cells. Accordingly with these results Hep 2C and Vero cells showed the same susceptibility to poliovirus type 1. In MCR-5 cells the average titre (G.M.T.) was 1 log10 below the average titres (G.M.T.) from Hep 2C or Vero cells. With poliovirus types 2 and 3 the higher titres were obtained on Hep 2C cells with significant differences to the titres observed in Vero and MRC-5 human diploid cells, which showed less susceptibility.