Abstract: Selenyl- and telluryl derivatives of steroids are prepared having in general the formula of 19-methylselenyl or 19-methyltelluryl derivatives respectively of cholesterol and related 3-oxy 5-.DELTA.-steroids, such as sitosterol and campesterol.More specifically these substances are prepared with radioactively labelled Se and Te atoms by first preparing compounds of the type of alkyl lithium selenides and tellurides respectively and related compounds and these are reacted with steroid compounds of the type stated having a smoothly transferable substituent in the 19 position.These labelled compounds are suitable for preparing diagnostic compositions for examination into internal organs such as the adrenal glands, applying scintigraph detection methods. They also can be applied for radio-immunologic determinations and other diagnostic determination methods.