The effect of protein antigens on the locomotion of lymphocytes from the lymph nodes draining the site of antigenic challenge in immunized mice, and from the same nodes in control mice, was studied in filters using a checkerboard assay in which the absolute concentration and the concentration gradient of attractant was varied in a series of chambers. Serum albumin (HSA or BSA) was chemokinetic for unimmunized lymphocytes inasmuch as the distance migrated into filters by cells in its presence varied with the absolute concentration of albumin, but not with the concentration gradient, indicating an influence of the serum albumin on the rate but not on the direction of locomotion. Ovalbumin and nonalbumin proteins did not show this effect. Using the same assay, the migration of primed lymphocytes in the presence of the priming antigen was shown to be influenced by the antigen gradient in a way that suggested a positive chemotactic response of the lymphocytes to antigen. This response was only shown clearly when the cells were in a chemokinetic medium containing serum albumin.