Testing plate made of optical quartz has been developed for the purpose of determination of sun protection factor (SPF)(in vitro) by the method of diffusion transmission spectroscopy; the plates were coarsened by sanding and grinding to surface roughness values (Ra) of 18 mum. The plate was coated with a film of sunscreen by an application of 2 mg cm(-2) as that used for determination of SPF(in vivo) by the COLIPA method. The transmission values measured were converted into the SPF(in vitro) and the protection factor in ultraviolet A light, UVAPF(in vitro). The testing plate was tested with commercial sunscreens. The found values of SPF(in vitro) fit well with the values determined by means of the COLIPA method in vivo. The plates coated with sunscreen film were irradiated with light simulating the sun radiation. The values of protection factors obtained before and after irradiation were compared, and the differences were used for estimation of photostability of the UV filters included.