Drops with different concentrations of 3':5'-cyclic AMP were deposited at various distances from small populations of Dictyostelium discoideum amoebae, and the distances over which 50% of the amoebae drops reacted positively were determined. The linear regression analysis of a bouble logarithmic plot of distance against concentration gives a straight line with a slope of 1/4.25, which value suggests that the amoebae respond to a spatial gradient of cyclic AMP concentration. The threshold value for the signal is 3.6 X 10(-9) M/mm with a sensitivity of measurement of about 1%. These findings are discussed in relation to our present knowledge of cyclic AMP receptors.