The present research investigated the time required for reconstructing normal microtubular pattern and the phenotype of cells after short-term (30 min.) and long-term (24 hrs.) pretreatment with the lectin concanavalin A (Con A). Short-term pretreatment led to the formation of incomplete tubules within 2 and 24 hrs. in cells cultured in 199 medium alone. The addition of serum to the medium reversed the globular phenotype and allowed the formation of normal microtubules, even after prolonged pretreatment with Con A. Whereas long-term Con A treatment provoked a reduction in DNA synthesis in 199 alone, in serum-added 199 the percentage of 3H-thymidine incorporation of treated cells tended to reach that of controls over time.