A new and sensitive technique is introduced for evaluating in vivo leukocyte chemotaxis. A small, disposable plastic chamber is used for holding the test solution, and an electric ink eraser for controlled abrasion of the skin test site. Another innovation is the use of capturing micropore membranes that, when placed between the test solution and the skin, collect cells for a permanent record. The small size of the chamber, the ease of application and immobilization, and the short time needed for cell collection permit experimentation with laboratory animals as well as with human volunteers. The accumulation of leukocytes in the chambers containing previously frozen or complement-activated autologous serum was 20 to 40-fold greater than in buffer controls after 3 h. Cell counts of approximately (1–2)×103 leukocytes per mm3 were obtained in 3 h when serum diluted 1∶1 with Hank's solution was applied to human or rabbit skin. Although predominantly polymorphonuclear leukocytes were detected in the chambers after just 3 h, prolonged incubations of 6–12 h found mononuclear cells also invading the chambers. These results clearly simulate the sequence of cellular emigration commonly observed in local inflammation.