Delayed-type hypersensitivity (DTH) testing represents a simple in vivo method for monitoring cellular immune responses. In a phase II clinical trial on HIV-infected patients (n = 38) of the peptide-based HIV-1 immunotherapy candidate Vacc-4x, we monitored DTH responses to three antigen concentrations of Vacc-4x. We have shown that DTH can be used quantitatively to measure immune reactions to Vacc-4x and its individual peptide components. Our data stress the qualities and differences of induration and erythema, both in discriminating individual antigens and in monitoring immunizations over time. The data also indicate that DTH baseline-status might have important impact on immunization kinetics.