Cystatins are physiological inhibitors of cysteine proteinases which are widely distributed in human tissues and fluids. In the present study we analysed both the cystatin activity and the different cystatin isoforms in gingival crevicular fluid and saliva samples of nine periodontitis patients. All crevicular fluid samples, which were collected with filter paper points, showed cystatin activity ranging from 7-67 units/mg protein. The mean cystatin activity (24 units/mg protein) was significantly lower (p < 0.05) than that of the saliva samples (mean 93 units/mg protein). The cystatin isoforms in the crevicular fluid were further characterized by immunoblotting with specific antibodies against cystatin C, S, SN and A. While they were clearly present in saliva, cystatin C, cystatin S and cystatin SN could not be detected in any of the crevicular fluid samples. Remarkably, cystatin A was found in all the crevicular fluids as well as in the saliva samples. It is concluded that the cystatin activity found in crevicular fluid is caused, at least partially, by cystatin A. Furthermore, the gingival crevicular fluid is not a major contributor of cystatin C, S and SN activity in saliva.