Chemotactic and chemokinetic migration of polymorphonuclear leukocytes in sera from patients with previous yersinia arthritis and from healthy subjects with or without HLA--B27 were studied by the leading front method. Irrespective of yersinia arthritis, zymosan-activated sera from subjects who were HLA--B27 positive were significantly more chemokinetic than were HLA--B27 negative zymosan-activated sera from healthy control subjects. The chemotactic activities of zymosan-activated sera that were HLA--B27 positive or negative, as determined by chemotactic increments, were much the same. The results suggest that zymosan-activated serum that is HLA--B27 positive stimulates random migration of polymorphonuclear leukocytes, but not their directional migration, more than does HLA--B27 negative serum that has been zymosan activated. This may contribute to accumulation of polymorphonuclear leukocytes at the site of inflammation in vivo and thereby to inflammatory symptoms in yersinia arthritis patients with HLA--B27.