The polycation polylysine, at different degrees of polymerization, was found to cause a marked inhibition of the conjugation process. Inhibition of conjugation by polylysine was highly dependent on the molecular weight of the polymer. When polylysine of a mol wt of 1,250 (degree of polymerization=6) was used, a concentration of 1.6 X 10(-5) M was required for a complete inhibition of conjugation, while only 2 X 10(-7) M of polylysine of a mol wt of 71,000 (degree of polymerization=340) was needed for the same effect. Polyaspartic acid prevented the inhibition of conjugation by polylysein. Chelators of bivalent metals such as O-phenanthroline (10(-3) M), EDTA (10(-3) M), and EGTA (5 X 10(-3) M) strongly inhibit the conjugation process in Tetrahymena pyriformis. The inhibition was partially prevented when bivalent metals such as Zn++, Fe++, and Ca++ were added together with the chelators. The lectin concanavalin A (25 mug/ml) completely prevented the conjugation process, while other lectins, such as phytohemagglutinin (500 mug/ml), soybean agglutinin (75 mug/ml) and wheat germ agglutinin (250 mug/ml) had no effect. Inhibition of conjugation by concanavalin A is completely reversible by 40 mM of alpha-methyl-D-mannoside.