AB3121 makes it possible for juveniles 16 years old or older to be declared unfit for juvenile court processing for having been charged with 1 of 11 specified serious offenses. To measure the impact of this law in Los Angeles and Alameda counties, data were collected on all juveniles who had fitness hearings in 1976 and 1977 (565 cases in Los Angeles County and 327 cases in Alameda County). Both counties had substantial increases in the number of fitness hearings and in unfit declarations in spite of decreases in arrests for serious juvenile crimes across the years studied. Conviction rates were about equal in the adult and juvenile courts in both counties, but confinement rates were higher in the adult courts. Defendants most likely to be waived in Los Angeles County were those charged with armed robbery and the destructive use of firearms as well as juveniles with a history of gang association. In Alameda County, the charges against waived juveniles were most often ''criminal driving,' strong-arm robbery, victimless crimes, and armed robbery. The legislation did not have a significant impact in other areas of the State. Tabular data and 1 reference.