Comparison between the Chemotactic Index determined by experiment (symbols and dashed lines) and our theory (solid curves). Chemotactic Index is defined as the distance moved by a cell in the direction of a gradient divided by the total distance moved. Experimental data were obtained by van Haastert and Postma () from Dictyostelium discoideum cells migrating toward pipettes containing four different cAMP concentrations, 0.1 μM (squares), 1 μM (triangles), 10 μM (diamonds), 100 μM (circles). The theoretical curves were obtained for a perfectly absorbing sphere by using a single fitting parameter Da3T = 1.2·105 μm5, corresponding to, e.g., a cAMP diffusion constant of D = 300 μm2/s, a cell radius of a = 5μm, and an averaging time T = 3.2 s, using the gradient profiles from ref. and the Chemotactic Index from . Experimentally, the Chemotactic Index only reaches approximately 0.9 at zero distance, so we rescale our theory curves by 0.9. (Inset) Chemotactic Index as a function of (cz0)2/c0 in units of nM/(μm)2, where cz0 and c0 are the gradient and concentration, respectively.