Sixty-four patients were light tested to determine the relationships between systemic and discoid lupus erythematosus (SLE, DLE) and polymorphous light eruption (PMLE) in immunopathologic responses to light and the mechanism of action of antimalarial drugs. The effects of intradermal hydroxychloroquine sulfate (Plaquenil), methylprednisolone sodium succinate (Solu-Medrol), methotrexate, and saline, and topical applications of hydroxychloroquine and saline on cutaneous responses to ultraviolet light were studied in a smaller group. There was significant diminution of delayed erythema and blocking of the reproduced eruptions almost only at the intradermal hydroxychloroquine site in five (SLE, 1; DLE, 2; and PMLE, 2) out of seven positive lighttest reactors. The incidence of positive light tests was higher in the antinuclear factor (ANF) tests (11.1%) suggesting a more active disease state in the ANF positive PMLE patients. Antimalarial drugs may conceivably act by interfering with complement activity at the local tissue level in SLE, DLE, and PMLE or by deoxyribonucleic acid binding.