Three early metabolic responses to wheat germ agglutinin (WGA), traditionally considered a nonmitogenic or anti-mitogenic lectin, were studied in human peripheral blood mononuclear cells and purified lymphocytes. Results were compared with those obtained with the known mitogens, concanavalin A (Con A) or Phaseolus vulgaris phytohemagglutinin (PHA). 1) Protein phosphorylation. Con A and WGA caused identical increases in [32P]-phosphate incorporation into a number of phosphoproteins in mononuclear cells and lymphocyte preparations prelabeled with [32P]-orthophosphate. The phosphorylation of a single cytoplasmic protein of m.w. 65,000 (p65) was particularly marked. Augmentation of phosphorylation occurred at similar lectin concentrations and followed the same time course, being maximal within 10 min with either lectin. Lectin stimulation of phosphorylation was inhibited by the appropriate haptenic sugars (N-acetyl glucosamine for WGA and α-methyl mannoside for Con A). 2) Aminoisobutyric acid (AIB) transport. Con A stimulated [14C]-AIB accumulation in both mononuclear cells and purified lymphocytes; however, WGA stimulated only isolated lymphocytes. The lymphocyte AIB responses to Con A and WGA were identical with respect to time course of induction, transport specificity, K(m), susceptibility to inhibition by haptenic sugars, and lectin concentration threshold, although the response to WGA was less marked at every concentration tested. The apparent absence of WGA responsiveness in mononuclear cell preparations can be explained by simultaneous decreases in AIB uptake in adherent cells superimposed on increases in transport in lymphocytes. 3) Biosynthetic labeling of particulate proteins. By using 2-dimensional polyacrylamide gel electrophoresis, WGA and Con A were shown to produce indistinguishable increases in the incorporation of [35S]-methionine into several lymphocyte particulate proteins of apparent m.w. 28,000 (p28) and 30,000 (p30). Protein p28 is known to be a cell-surface glycoprotein. Again, this response was blocked by appropriate sugars. These results demonstrate that WGA rapidly stimulates human lymphocytes, producing biochemical changes that are qualitatively identical and quantitatively similar to those caused by Con A in regard to at least 3 parameters of early activation. The absence of more marked DNA synthetic and complete proliferative responses in human lymphocytes stimulated with WGA may be due to delayed inhibitory or cytotoxic effects of WGA on monocytes and perhaps subpopulations of lymphocytes as well.