Activation of p21ras as measured by conversion of rasGDP to rasGTP following CD4XL. Labeling with ³² P-orthophosphate, cross-linking with gp160 and anti-gp160, cell lysing, and immunoprecipitation was performed as described in Materials and Methods. Ras bound GDP and GTP were eluted and analyzed by thin layer chromatography, visualized by autoradiography, and quantitated by densitometry. Conversion of rasGDP to rasGTP was calculated as a ratio of percent rasGTP/GDP:GTP. Data represents mean of two experiments.