Systematic studies were carried out on two different strains (Gatersleben and Cairo) of Hyoscyamus muticus L. (Solananaceae) in order to analyse the variation in the contents of the two main tropa-alkaloids in individual plants and protoplast-derived cell culture clones. The hyoscyamine content was determined by radioimmunoassay, the scopolamine content by radio- and enzymeimmunoassay and the total tropane alkaloid content by quinuclidinyl benzilate assay. The development stage of the plant is important for alkaloid production. Clear maximal foliar scopolamine and hyoscyamine contents were reached at the onset of flowering and during the full blooming stage, respectively. In the roots the changes in the production of these alkaloids were not considerable. Hyoscyamine is a major alkaloid in the plants as well as in the cell culture clones. In a few exceptions the scopolamine content was greater than that of hyoscyamine, but this phenomenon was not inherited by the cultured clones derived from these plants. High and low tropane alkaloid production was inherited by the selfed F1, generation plants. Great variability in these alkaloids was observed among individual plants or clones in the same plant or clone population. There was a significant difference between the Cairo strain and the Gatersleben strain as regards their ability to produce tropane alkaloids. Haploid plants of both strains contained more hyoscyamine and scopolamine than the diploid ones. Hyoscyamine and scopolamine were the main alkaloids in the clones and in the plants. Via QNB-assay an interesting clone was found which contains remarkable amounts of unknown tropane-group alkaloids.