Experimental infection of monkeys with the IC-B strain of measles virus (MV), which was isolated in marmoset B lymphoblastoid B95a cells from an acute measles patient, caused clinical signs typical for measles, while infection by the IC-V strain isolated in African green monkey kidney Vero cells from the same patient did not cause any clinical signs in infected monkeys. The IC-B strain replicated only in B95a cells, whereas the IC-V strain replicated in both B95a and Vero cells (3,6). To clarify which gene or mutation(s) was responsible for the difference in these phenotypes, the nucleotide sequences of the entire genomes of the IC-B and IC-V strains were determined. Comparative nucleotide sequence analyses revealed only two nucleotide differences, one in the P/V/C gene and the other in the M gene, predicting amino acid differences in the P, V and M proteins and a 19 amino acid deletion in the C protein of the IC-V strain. The truncation in the C protein was confirmed for the IC-V strain by immunoprecipitation using the C protein specific antiserum. No nucleotide difference was found in the envelope H gene. These results indicated that nucleotide difference(s) in the P/V/C or/and M gene, and not H gene, was responsible for the different cell tropism and pathogenicity of MV in this case.