Phase I involved testing of 104 males from the Deuel Vocational Institute in Stanislaus, Calif., and 83 controls. This phase did not replicate the findings of others who reported high levels of lead, cadmium, and copper in violent offender; however, high levels of manganese were found in inmates but not in control groups. Phase II tested 60 male inmates from the Los Angeles and San Bernardino County jails (Calif.) and 30 male controls. In this phase, the possibility of artifactual results arising from prison cooking utensils was controlled for by sampling early after incarceration. Phase III tested 29 male inmates from the San Bernardino County Jail and 59 male controls. This phase was included to substantiate the initial post hoc findings in an additional jail population. In both latter phases, significantly elevated manganese levels were found in the hair of violent compared to nonviolent subjects. A review of the effects of manganese at deficient and toxic levels does not provide a simple answer as to why manganese levels are elevated in the hair of persons incarcerated for violent behavior. The study does not implicate the prison environment or soaps and shampoos used in California prisons. Other factors, such as alcohol, dietary, or psychosocial factors, might influence manganese levels in hair, or any of these factors might function in combination with mild manganese toxicity to contribute to aberrant behavior. 1 table, 1 figure, and 40 notes