The relationship of sun protection factor (SPF) to minimal erythema dose (MED), Japanese skin type, and skin color was investigated on the unexposed backs of 48 healthy subjects. SPF of a UVB-sunscreen was determined using sun lamps as a light source. A significant correlation was found between SPF and MED; subjects with lower MED showed higher SPF. The average SPF decreased with increasing skin type. There was no significant correlation between SPF and skin color (L*a*b* color system).