Intracellular phosphotyrosine content and integrin 4 chain surface expression in thymocytes from control or day 2¹/₂ P-Ir mice. Plots show PTyr and integrin 4 chain levels according to the level of CD3 expression as defined in the histograms. Background labeling with irrelevant antibodies (Irrel. Ab) is also shown. Values were obtained with the labeling of pooled thymocytes from at least three animals per point.