HLA-A and -B antigens are resistant to ultraviolet (UV) light whereas HLA-D, as defined by the mixed leukocyte culture reaction (MLR), is sensitive to UV light in initial or first culture. In primed LD typing, the HLA-D responsible for stimulation is relatively resistant to UV light in second culture. We have developed a method for evaluating a specific proliferative response to DNP in dinitrochlorobenzene (DNCB)-sensitized human subjects and found a requirement for a component of autologous leukocytes to activate DNP-responsive lymphocyte clones. Autologous DNBSO3-coupled leukocytes with and without UV were used to stimulate lymphocytes in DNCB-sensitized subjects. 3HTdR incorporation was measured after 4 or 5 dys in 1st or “primary” culture. After 9 to 14 days, blast-derived lymphocytes were set up in 2nd or “secondary” cultures and exposed to the same autologous DNBSO3-treated leukocytes with and without UV. Accelerated 3HTdR incorporation was measured in these secondary clonally primed cultures after 20 to 44 hr. The results showed that UV light abrogated the antigen-presenting or stimulator function in the 1st culture and diminished, but did not abrogate, the antigen-presenter function in the 2nd culture. These findings indicate that the DNP-antigen presenting function is behaving in a manner similar to that observed for mixed leukocyte culture determinants in 1st and 2nd cultures.