In our previous observation, the T-antigen in unclond mouse embryo cells transformed by SV 40 or SV 40 DNA declined gradually through long term cultivation. To analyze whether this phenomenon is due to selection or reversion and/or conversion, we observed the T-antigen of a series of clones derived from a single cell in the SV 40-transformed cells by the immunofluorescent method. The T-antigen was either positive or negative in all cells in each clone, and none of the clones was consisted of the mixture of T-antigen positive and negative cells. The T-antigen negative cells were more predominant in the rate of growth than the T-antigen positive ones in this mouse embryo cells. One reasonable explanation on the data may be that the decline of T-antigen in the uncloned SV 40-transformed cells is due to the selection of the T-antigen negative cells. This fact may be one of the reason for difficulty in establishing the SV 40-transformed mouse embryo cell lines.