Purpose: To examine the effects of intensity and duration of exercise stress on induction of in-vivo immunity in humans using experimental contact hypersensitivity (CHS) with the novel antigen Diphenylcyclopropenone (DPCP). Methods: Sixty-four healthy males completed either 30 minutes running at 60% V[spacing dot above]O2peak (30MI); 30 minutes running at 80% V[spacing dot above]O2peak (30HI); 120 minutes running at 60% V[spacing dot above]O2peak (120MI) or seated rest (CON). Twenty-minutes later subjects received a sensitizing dose of DPCP and four-weeks later the strength of immune reactivity was quantified by measuring the cutaneous responses to a low, dose-series challenge with DPCP on the upper inner-arm. Circulating epinephrine, norepinephrine and cortisol were measured pre, post and 1h post-exercise or CON. Next, to better understand whether the decrease in CHS response on 120MI was due to local inflammatory or T-cell mediated processes, in a cross-over design, eleven healthy males performed 120MI and CON and cutaneous responses to a dose-series of the irritant, croton oil (CO) were assessed on the upper inner-arm. Results: Immune induction by DPCP was impaired by 120MI (skin-fold-thickness -67% vs CON; P