Los Angeles Basin smog, caused principally by automobiles, consists of ozone, oxides of nitrogen, carbon monoxide and peroxyacyl nitrates plus some fluorides and sulfur oxides. It causes much leaf injury to leafy vegetables, grapes and ornamentals in this basin and elsewhere; citrus is very resistant to this kind of damage. Because little overt injury was seen on citrus but yields were continuing to decline, a unique, broadly based cooperative effort was begun in 1960 to find out if and, if so, how much actual injury was being caused by air pollutants.