Smoking, response to allergen skin testing, and nonspecific airways reactivity in students entering a career program for animal health technicians (AHT) were studied at their entrance and 7 months into the program to determine whether such persons provide a suitable cohort to overcome the selection biases accompanying investigations of occupational asthma. Previous occupational exposure to animals (65%) was associated positively with allergic symptoms but negatively with skin response to animal allergens and to airway hyperreactivity (AR). AHTs remaining in the program were more likely than those dropping out to have (1) worked with animals, (2) positive skin responsiveness to animal allergens, and (3) AR; the latter was significantly associated with positive skin-test responses to animal allergen testing. This study demonstrates that significant exposure to animals may have occurred among workers entering animal-handling careers. Additionally, competing "healthy" and "resistant" worker effects operate among AHTs to influence the prevalence of occupational asthma in this population.