This study was undertaken to investigate the efficacy of 5% PABA cream using mouse ear swelling reaction(ESR). Mice were exposed to 100mJ/cm of UVB, five times a week for four weeks, on the both ventral aspect of the ear, with application of 5% PABA cream on the right ear. The results were as follows : 1. The intensity of ear swelling reaction of 5% PABA protected group was reduced greater than unproteeted group after the first 3 days of UUR. 2. The intensity of ear swelling reached at peak after 1 week of the ultraviolet radiation. Thereafter it has decreased gradually the following 4 weeks. The difference of ear swelling between the two groups was the greatest after 1 week, and the sunscreening efficacy of 5% PABA cream has remained persisted for 4 weeks. 3. The number of mice which have shown severe inflarnmatory response after ultravioiet radiation was more in unprotected group than that in 5% PABA protected group. 4. Determination of mouse ESR is considered a good method for the evaluation of longterm efficacy of sunscreen preparation. (Kor J Dermatol 28(4): 419 424, 1990)