AbstractEvidence has been presented to show that chemokinesis and chemotaxis of human neutrophil granulocytes can be controlled by distinct cellular mechanisms and by different chemicals. Materials such as human serum albumin or fibrinogen had chemokinetic but no chemotactic properties. Highly purified preparations of serum‐derived peptides containing classical anaphylatoxin had detectable chemotactic activity only. Chemokinetic as well as chemotactic substances were required for the expression of chemotaxis in the form of efficient directional locomotion. The roles of chemokinesis and chemotaxis in the regulation of directional locomotion have been analyzed.Further experiments showed that the chemotactic response conforms to the law of Weber‐Fechner. The number of cells which had accumulated in response to a chemotactic stimulus was proportional to the logarithm of the concentration of the chemical substance. Readaptation from high to low chemotactic stimulus intensities occurred only to a limited ext