A strain of Vesicular Stomatitis virus, serotype Indiana (VSV), was compared after a single in vitro passage and after 16 in vitro passages in human white blood cells from patients suffering mostly from acute myeloid leukemia. The in vitro growth characteristics of these two virus lines in white blood cells (predominantly myeloblasts) from a patient with acute myeloid leukemia were measured, starting with low multiplicities of infection. No differences were seen in the following parameters: Length of latency phase, growth rate, final titer, thermal inactivation rate, plaque size, titer difference in chick embryo vs. human diploid cells, and extent of neutralization by a standard rabbit antiserum. Infectivity titrations in chick embryo cells gave consistently about ten times higher values than the same titrations done in human diploid cells, strain Wi38. More than 1,000 infectious units of VSV (as measured in chick embryo cells) were produced per myeloblast within 44 h. It was concluded that a single passage was sufficient to fully adapt VSV to human myeloblasts.