Confluent cultures of normal primary human skin fibroblasts were incubated with various glucocorticosteroids which are in current use clinically for the treatment of various skin disorders. For all steroids concentrations were found at which collagen hydroxyproline formation was inhibited, while total protein synthesis was little affected. The concentration effective for inhibition was highest for hydrocortisone and lowest for clobetasol-17-propionate. All other steroids (hydrocortisone-17-butyrate, triamcinolone acetonide and betamethasone-17-valerate) showed medium effectiveness. Fluorination as such was not a factor in the degree of inhibition. The inhibition observed was shown to be independent of concomitant specific effects on cell proliferation or cell turnover. The possible implications of these findings on the therapeutic effects in psoriasis and the frequently occurring atrophic side-effects of these steroids are discussed.