AbstractThe glycerinated stalks of the peritrich ciliate,Vorticella, can contract helically and reversibly on the addition of not only Ca2+but also other divalent or trivalent cations having ionic radii not far from 1 Å. In order to investigate the stalk contraction quantitatively in the absence of Ca2+‐chelators, we developed a method to eliminate contaminating Ca2+and other metal ions in KC1 and pHbuffer solutions by using a Ca2+‐and heavy metal ion‐specific ion exchange resin (Eporas MX‐2) Thus, it was possible to measure the relationship between the fractional stalk length ofVorticellaand the free concentration of alkaline earth metal, transition metal, and lanthanide ions in the 0.1 M KC1 and buffer (pH 6.8) solutions. Among these ions, Ca2+, Nd3+, and Eu3+(having ionic radii of about 1 Å) had the highest affinity for the contractile element in the spasmoneme. As the concentration of lanthanide ions (except Nd3+and Eu3+) is increased, theVorticellastalk contracts abruptly at a threshold level; this means that the Hill's parameter is very high, probably more than 6. The results of these experiments and of the co‐mixtures of Ca2+and Tb3+suggest that a contractile element in the spasmoneme contains both contractile Ca2+‐binding and regulatory Ca2+