We conducted a cross-sectional study of tuberculin reactivity among residents of two northern California migrant-farm-worker housing centers. Participants completed a brief health questionnaire and were offered tuberculin skin testing with radiologic and medical follow-up. Four hundred and sixty-nine persons (estimated participation rate: 70%) completed questionnaires. All but one were Hispanic. Two hundred and ninety-six (63%) participants completed tuberculin skin testing and 49 (16.6%) showed reactivity (> or = 10 mm induration at 48-72 hours). Increased prevalence was seen for the 15-39-year age group (vs. persons younger than 15: OR 2.59; 95% CI 0.79-8.47), former smokers (vs. never smokers: OR 3.11; 95% CI 1.20-8.09), and persons born outside the U.S. (OR 2.09; 95% CI 0.66-6.61). Prophylaxis with isoniazid was recommended for 23 persons; nine (39%) completed therapy. No cases of active tuberculosis were found. Prevalence of tuberculin reactivity in this population is lower than reported among Hispanic farm workers in the eastern and midwestern U.S. Higher prevalence may obtain among California farm workers not included in the study population, including homeless, single, and highly mobile persons. Public-health efforts in this population should focus on ever-smokers, young adults, and persons born outside the U.S.