Radioimmunoassay results from 13 criminalistics laboratories in the State indicated a 16 percent overall incidence of delta 9-tetrahydracannabinol (9-THC) in the blood samples. Where no alcohol was present in the blood samples (185 of the total population), the incidence of 9-THC rose to 24 percent. Marijuana use crossed a wide range of ages. Licensed drivers between 30 to 60 years old demonstrated a slightly higher incidence of use (19 percent) than did those between 14 and 29 years of age (13 to 15 percent). Incidence varied by County of arrest, from a low of 6.7 percent in Butte County to a high of 38 percent in Calaveras County. The range of incidence by County of residence ranged from 4 percent for Butte County to 31 percent for Alameda County. All persons in the impaired driving population whose blood samples contained 9-THC failed the roadside sobriety test. Blood levels of the drug were not particularly high (median 9ng/ml). These consistently low levels may be attributable to the fact that 9-THC disappears from the blood in 2 to 4 hours and there is a delay from time of arrest to the time the officer is able to get the sample to the laboratory. In controlled tests, subjects showed signs of impairment on the roadside sobriety test 2.5 hours after smoking, yet showed blood levels below detectable limits of the radioimmunoassay technique. 32 figures, 8 attachments, and 15 references.