Distributional changes of individual mRNAs between free ribonucleoprotein particles (mRNP) and ribosome-bound transcripts are used to assess translational control. Simultaneous analysis of many mRNA species is required to estimate the overall contribution of translation to the regulation of gene expression. To this purpose, total cytoplasmic RNA was fractionated in sucrose step gradients and poly(A)+ RNA was prepared from mRNP and ribosome-bound fractions. Since direct, simultaneous analysis of a profusion of mRNAs is not feasible, distribution of their in vitro translation products was examined after separation in 2-dimensional gels, followed by computer-based analysis of autoradiographs. When this analysis was applied to antigenically stimulated T cells, 36% of in vitro translation products showed a greater than 10-fold increase in intensity, suggesting transcriptional activation of the corresponding mRNAs. In comparison, 7.9% of individual mRNAs (54 of 685 species) were translationally activated. They were redistributed from free mRNP to ribosome-associated fractions; 4.7% (32 species) were translationally repressed, as indicated by the opposite pattern. The differential recruitment of 12.6% of mRNA species demonstrates specificity and the general significance of translational control during T cell activation, which implies that translation may play a similar role in regulating gene expression in a variety of physiological processes.