The size and production of polymorphonuclear leukocyte (PMN) chemotactic factors by Propionibacterium acnes has been studied. All eight strains of P. acnes which were tested liberated PMN chemotactic factors in their growth culture media. The factor(s) produced by one strain, 6919, were studied in greater depth. The PMN response was proportional to the dose of culture supernatant and chemotactic activity increased with the duration of P. acnes culture. The neutrophil migration towards culture supernatants was chemotactic, with a slight stimulation of random migration by 72-h supernatants. The size of the chemotactic molecules was studied through dialysis, ultrafiltration, and Sephadex G-25 chromatography of 72-h supernatants. Low-molecular-weight chemotactic factors were found to be predominant by each method of size determination.