The interaction between the sunscreen agent, 4-methylbenzylidene camphor (4-MBC) and hydrophilic alpha-, beta- and gamma-cyclodextrin derivatives was investigated in water by phase-solubility analysis. Among the studied cyclodextrins, random methyl-beta-cyclodextrin (RM-beta-CD) had the greatest solubilizing activity. The complexation of the sunscreen agent with RM-beta-CD was confirmed by nuclear magnetic resonance spectroscopy and powder X-ray diffractometry. The light-induced decomposition of 4-MBC in emulsion vehicles was markedly decreased by complexation with RM-beta-CD (the extent of degradation, determined by HPLC, was 7.1% for the complex compared to 21.1% for free 4-MBC). The influence of RM-beta-CD on the human skin penetration of the sunscreen was investigated in vivo using the tape stripping method, a useful procedure for selectively removing the outermost cutaneous layers. Considerable quantities (21.2-25.1% of the applied dose) of 4-MBC permeated in the stratum corneum. However, no significant differences in the amounts of UV filter in the 10 first strips of the horny layer were observed between the formulations containing 4-MBC free or complexed with RM-beta-CD. Therefore, RM-beta-CD complexation did not alter the retention of 4-MBC in the superficial layers of the stratum corneum, where its action is more desirable.