Surface cultures of 15-19 day old fetal mouse brain that do not initially express the Thy 1 antigen develop the antigen between 4 and 11 days in culture. The antigen can be detected by immunofluorescence of live cells on the culture plate on two morphologically distinct types of cells. Both neuronal-looking cells and a more general category of cells which includes fibroblastic cells can express the Thy 1 antigen. The antigen is not expressed on certain cells of the glial type. This method enables one to distinguish between different types of brain cells in culture both morphologically and immunologically at the same time.