Motile models ofAmoeba proteus prepared by extraction at −20 °C with a 50% buffered glycerol solution showed remarkable contraction on addition of Mg-ATP with retainment of Ca++ sensitivity. The initial contraction around the nucleus occurred on addition of Mg-ATP independently of the Ca++ concentration, and was followed by contractions of three different patterns. In 10−8M Ca++, the granuloplasm contracted as a whole and separated from the membrane leaving no detectable particles in the anterior region. In 10−6M Ca++, the granuloplasm contracted leaving some granules which showed active and vigorous two-directional streaming similar to that in the living cell. Sometimes a part of the plasma membrane twitched. The streaming lasted up to 20 minutes. In 10−4M Ca++, after the initial contraction around the nucleus, no significant movement was observed. All these movements in the models were inhibited by cytochalasin B or N-ethylmaleimide. The relationship between the Mg-ATP and the Ca++ concentrations was examined.