Antigenic properties of Getah virus were monitored during 80 serial passages in Vero tissue cultures. The parent virus lost its serotype identity by passage 80. A clone derived from the parent remained antigenically stable during passage. The experimentally induced antigenic alterations in Getah virus were attributable to the tissue culture host selection of constituent subpopulations of virus with properties unlike those of the original parent. The marked differences in neutralization titers of low- and high-passage virus were considered related to or influenced by (i) the variation in subpopulation selection of the virus during passage and (ii) the variation in availability of antigenic stimulants during preparation of antiserum.