It was previously reported that the lectins wheat germ agglutinin (WGA) and concanavalin A (Con A) inhibit the mitogenic actions of multiple peptide growth factors in human fibroblasts without having a significant effect on mitogen binding. The current studies were designed to further examine the mechanisms of this antimitogenic action of lectins. Addition of WGA at progressively later times after stimulation of fibroblasts with peptide mitogens revealed significant inhibition of DNA synthesis even when the lectin was added 16-20 h after growth factors. This suggests an inhibitory effect on a pathway occurring late in G1, or close to the G1/S boundary. WGA also inhibited stimulation of DNA synthesis by non-peptide agents such as colchicine and vanadate ion, indicating that the lectin inhibits a common distal step in the mitogenic response, rather than acting primarily on events occurring at the level of the growth factor-receptor interaction. WGA had a rapid (within 30 min) inhibitory effect on insulin-stimulated amino acid uptake, but Con A, which like WGA blocked mitogen-stimulated 3H-dT incorporation, had little effect on stimulation of amino acid uptake. Thus the inhibition of DNA synthesis and amino acid uptake by lectins appear to be mediated by distinct mechanisms. WGA binding to fibroblasts persisted even when the lectin was removed from the incubation medium, but unlike 125I-EGF, which was rapidly internalized at 24 degrees C, little 125I-WGA was internalized. Incubation of fibroblasts for 20 h with WGA or Con A was not toxic to cells, since reversal of lectin binding by the appropriate saccharide allowed normal subsequent stimulation of DNA synthesis by EGF and insulin. However, the observation that cells exposed to antimitogenic lectins undergo a marked decrease in cell spreading suggests that changes in cell shape may be relevant to the mechanism by which lectin-treated fibroblasts become unresponsive to mitogenic stimulation.