Immunoprecipitation of Fas protein in 3DO cells: untreated (lane 2), treated with anti-CD2 (5 μg/well) plus anti-CD3 (1 μg/well; lane 3), anti-CD2 (5 μg/well) alone (lane 4), or anti-CD3 (1 μg/well) alone (lane 5) for 20 hours. Cells were also immunoprecipitated with isotype-matched antibody (lane 1). Densitometric analysis (relative units) of the film is also reported at the bottom of the figure.