The factor which inhibits cephalic regeneration has previously been partially purified. It is a peptidic substance (molecular weight approximately 2 000). On the other hand, the factor responsible for clitellum turgescence is found in the same fractions as the peptidic inhibitory factor. After preparative electrophoresis at pH 3.9, both factors coexist in fraction C, biologically active (Fig. 1 and Table I). After HPLC, fraction D inhibits the cephalic regeneration and keeps clitellar structures (Fig. 2 and Tables II and III). The clitellogenic factor is hydrolysed by proteolytic enzymes. Thus, this factor is peptidic like the cephalic regeneration inhibitor (Table IV). Fraction D is not completely purified (Fig. 3). However, it is likely that both factors are one single neurohormone comparable to neuropeptides found in vertebrates.