IN A 1974 STUDY ZANDER ADDRESSED THIS QUESTION BY STUDYING RECORDS FROM THE OLD BAILEY AND THE INNER LONDON CROWN COURT FOR 200 ACQUITTALS AND 1,235 CONVICTIONS. BASED ON PRIOR CRIMINAL RECORD, EACH DEFENDANT WAS CLASSIFIED AND COURT OUTCOMES FOR THE CURRENT CHARGE WERE ASSESSED. RESULTS INDICATED THAT ACQUITTAL RATES ARE LOWEST FOR THOSE WITH NO PRIOR RECORD, AND THE CHANCES OF ACQUITTAL DECREASE AS THE DEFENDANTS' PRIOR INVOLVEMENT WITH THE LAW INCREASE. FROM THIS IT WAS CONCLUDED THAT PROFESSIONAL CRIMINALS (DEFINED AS THOSE WITH PRIOR CONVICTIONS AND SENTENCES OVER 1 YEAR) ARE NOT UNDULY FAVORED BY THE COURTS AND DO NOT GET OFF COMPARATIVELY LIGHTLY WHEN THEY COME UP FOR TRIAL. A MAJOR DIFFICULTY WITH THE ZANDER STUDY IS THAT A SUBSTANTIAL CRIMINAL RECORD IS NEITHER A NECESSARY NOR A SUFFICIENT CONDITION FOR DETERMINING PROFESSIONAL CRIMINAL STATUS. TO REMEDY THIS PROBLEM, CRIMINAL INTELLIGENCE RECORDS OF SCOTLAND YARD WERE CONSULTED AND A LIST WAS MADE OF ALL CRIMINALS IN WHOM THE POLICE HAD MAINTAINED A CONTINUOUS INTEREST BETWEEN 1962 AND 1971. ON THE BASIS OF INTERVIEWS WITH KNOWLEDGEABLE OFFICERS AND DETECTIVES, THIS LIST WAS DIVIDED INTO A MAIN GROUP OF PROFESSIONAL CRIMINALS AND A LESSER GROUP, WHO ALTHOUGH OPERATING FULL TIME, ENGAGED IN SMALL SCALE ACTIVITIES REQUIRING ONLY MODERATE SKILLS. 'SMALL FRY,' A THIRD GROUP CONSISTING OF PETTY OPERATORS, WERE ALSO INCLUDED FOR ANALYSIS. WHILE THE SMALL FRY SPEND LESS TIME IN PRISON THAN THOSE IN THE MAIN OR LESSER GROUP, DATA ALSO INDICATE THAT THE SMALL FRY ARE LESS CONTINUOUSLY ACTIVE, ENGAGE IN LESS DANGEROUS AND PROFITABLE ACTIVITY, AND RESTRICT THEMSELVES TO LESS DETECTION-PRONE LOW-RISK ACTIVITIES THAN THE OTHER TWO GROUPS. HOWEVER, A COMPARISON OF SYSTEM-HANDLING SCORES FOR THE GROUPS, COMPUTED ON THE BASIS OF THE RATIO OF TOTAL CHARGES MADE TO CHARGES NOT RESULTING IN CONVICTION INDICATES THAT THE MAIN GROUP FARES BEST, FOLLOWED BY THE LESSER GROUP, WHILE THE SMALL FRY FARE WORST. FINALLY, A FURTHER ANALYSIS OF SUBGROUPS WITHIN THE MAIN OR PROFESSIONAL CRIMINAL GROUP IS PRESENTED. FINDINGS SHOW CONSIDERABLE VARIABILITY IN THE PROPORTION OF YEARS AT RISK AND PRISON-FREE AND SYSTEM-HANDLING ABILITY FOR THE SIX PROFESSIONAL CRIMINAL SUBGROUPS DEFINED AS ORGANIZERS, RECEIVERS, PROVIDERS, THIEVES, HEAVIES, AND VIOLENTS. THE HIGH PROPORTION (UP TO 91 PERCENT, AFTER EXCLUDING THOSE NEVER CONVICTED OVER A 10-YEAR PERIOD) OF PRISON-FREE TIME ENJOYED BY A MAJORITY OF THESE SUBGROUPS DESPITE PRESUMABLY CONTINUOUS OPERATION IS NOTABLE. IMPLICATIONS ARE DISCUSSED, AND THE COMPLEXITY OF CROSS-GROUP COMPARISONS OVER TIME ARE FURTHER EXPLORED. EXTENSIVE NOTES ARE INCLUDED. (JAP)