Previous studies have demonstrated that low doses of ultraviolet B (UVB) radiation (100 J/m2) abrogate the accessory function of freshly isolated murine epidermal Langerhans cells (fLC) and cause a parallel decrease in the ability of LC to express increased amounts of ICAM‐1 (CD54) in vitro. We have subsequently observed that the accessory cell function of cultured LC (cLC), as assessed by their ability to support anti‐CD3 monoclonal antibody (mAb)‐induced T cell mitogenesis, was not inhibited by levels of UVB exposure (100 J/m2) that completely inhibited the function of fLC, although exposure of cLC to UVB radiation (100 J/m2) resulted in a decrease in the level of ICAM‐1 expression on most cLC and a concomitant decrease in cLC survival during a subsequent 24‐h incubation. Time course studies revealed that T cells stimulated with anti‐CD3 mAb in the presence of cLC became committed to proliferate 4‐8 h after culture initiation, while 24–30 h of co‐culture was required for irreversible T cell activation when fLC were utilized as accessory cells. In addition, paraformaldehyde (PFA)‐fixed (non‐viable) cLC supported anti‐CD3 mAb‐induced T cell proliferation, whereas PFA‐fixed fLC were ineffective. We propose that cLC are functionally resistant to low doses of UVB radiation and chemical fixation because cLC express sufficient levels of the adhesion or co‐stimulatory molecules [including ICAM‐1 and Mac‐1 (CD11b/CD18)] required to induce T cell activation. Conversely, fLC are sensitive to the effects of UVB radiation and chemical fixation because these physicochemical agents prevent acquisition of critically important surface molecules in culture.