MARIJUANA ARREST DATA FOR JULY 1 THROUGH DECEMBER 31, 1970, WERE OBTAINED FROM POLICE RECORDS IN COOK COUNTY (CHICAGO), ILL., DOUGLAS COUNTY (OMAHA), NEBR., AND WASHINGTON, D.C. MARIJUANA USE ESTIMATES WERE OBTAINED IN SAMPLE SURVEYS CONDUCTED IN THE SAME JURISDICTIONS. ARREST PROBABILITIES WERE THEN DETERMINED FOR THE FOLLOWING GROUPS: MALES AND FEMALES; WHITE-COLLAR WORKERS, BLUE-COLLAR WORKERS, AND STUDENTS; BLACKS AND NONBLACKS; AND PERSONS UNDER AGE 25 AND OVER AGE 25. THE ANALYSIS SHOWS THAT ARREST PROBABILITIES ARE HIGHER FOR MALES THAN FOR FEMALES AND HIGHER FOR BLUE-COLLAR WORKERS THAN FOR OTHERS IN ALL THREE JURISDICTIONS. IN TWO JURISDICTIONS, YOUNGER PERSONS HAVE HIGHER ARREST PROBABILITIES THAN OLDER PERSONS. IN ONE JURISDICTION, ARREST PROBABILITIES ARE HIGHER FOR BLACKS THAN NONBLACKS. THE PLAUSIBILITY OF DIFFERENTIAL ENFORCEMENT AND DIFFERENTIAL VISIBILITY EXPLANATIONS FOR OBSERVED ARREST PROBABILITY VARIANCES IS EXPLORED BY EXAMINING DATA ON THE CIRCUMSTANCES OF MARIJUANA ARRESTS AND ON THE CHARACTERISTICS OF LAW ENFORCEMENT ACTIVITIES PRECEDING THE ARRESTS. BOTH INTERPRETATIONS APPEAR TO BE CONSISTENT WITH THE DATA. THE SOCIAL LOCATION OF MARIJUANA USE (I.E., THE CONTEXT AND MANNER IN WHICH IT IS USED) AS WELL AS SUSPICION CUED BY CERTAIN SOCIODEMOGRAPHIC CHARACTERISTICS OF MARIJUANA USERS ARE EQUALLY TENABLE EXPLANATIONS OF ARREST PROBABILITY DIFFERENTIALS. SUPPORTING DATA AND A LIST OF REFERENCES ARE INCLUDED. (AUTHOR ABSTRACT MODIFIED).