Propylene was administered by inhalation, 7 hours daily, 5 days weekly, at a concentration of 5000, 1000, 200 and 0 ppm, to Sprague-Dawley rats and Swiss mice. Groups of 120 male and 120 female rats (high-dose and controls) or 100 male and 100 female rats (mid- and low-dose) were treated for 104 weeks, and groups of 100 male and 100 female mice (each dose and controls) for 78 weeks. The animals were kept under observation until spontaneous death. Under the tested experimental conditions, the monomer was not shown to be carcinogenic.