THE STUDY SUBJECTS WERE WORKING-CLASS WHITE MALES INCARCERATED AT THE PATUXENT INSTITUTION IN MARYLAND. THEIR AGES RANGED FROM 19 TO 32 (MEAN, 28 YEARS). ALL HAD IQ'S (INTELLIGENCE QUOTIENTS) OF AT LEAST 90. ANALYSIS OF PLASMA TESTOSTERONE LEVELS OVER A 2-WEEK PERIOD SHOWED CONSIDERABLE STABILITY WITHIN INDIVIDUALS AND HIGHLY SIGNIFICANT DIFFERENCES BETWEEN INDIVIDUALS. THE MEAN LEVEL WAS CLOSE TO THAT REPORTED FOR THE INMATES' AGE GROUP. PLASMA TESTOSTERONE LEVELS DID NOT DIFFER IN FIGHTING AND NONFIGHTING INDIVIDUALS. HOSTILITY, ANXIETY, AND SOCIAL DESIRABILITY TEST SCORES CORRELATED WITH NEITHER TESTOSTERONE LEVEL NOR FIGHTING BEHAVIOR, ALTHOUGH THERE WERE SIGNIFICANT CORRELATIONS AMONG PSYCHOLOGICAL TEST SCORES. THE 10 INMATES WHO HAD COMMITTED PARTICULARLY VIOLENT, AGGRESSIVE CRIMES IN ADOLESCENCE HAD SIGNIFICANTLY HIGHER LEVELS OF TESTOSTERONE THAN THE 11 PRISONERS WITHOUT HISTORIES OF UNUSUAL VIOLENCE. THE FINDINGS SUGGEST THAT, WITHIN A POPULATION THAT IS PREDISPOSED TO ANTISOCIAL BEHAVIOR BY VIRTUE OF SOCIAL FACTORS, TESTOSTERONE LEVEL MAY BE AN IMPORTANT FACTOR IN DETERMINING WHETHER THAT BEHAVIOR WILL BE VIOLENT OR AGGRESSIVE IN ADOLESCENCE. SUPPORTING DATA AND A LIST OF REFERENCES ARE INCLUDED. (LKM)