Novel sterol derivatives of formula (I), in which: R.sup.1 represents a hydroxyl group or protected hydroxyl group, R.sup.2 represents a hydrogen atom and a double bond is present at c, or R.sup.1 and R.sup.2 together represent an oxo group and a double bond is present at b or double bonds are present at a and b; R.sup.3 represents a methyl group having .alpha.- or .beta.-configuration; R.sup.4 and R.sup.5, which may be the same or different, are selected from hydrogen atoms and aliphatic, cycloaliphatic, araliphatic and aryl groups, or together with the nitrogen atom to which they are attached form a heterocyclic group; and X represents a polymethylene group containing 2 5 carbon atoms, an oxa group-containing analogue thereof in which a methylene group other than that attached to the --CO.NR.sup.4R.sup.5 moiety is replaced by an oxygen atom, or an unsaturated analogue thereof containing up to two double bonds. Active compounds of the invention exhibit potent effects on the modulation of cell growth and differentiation and possess an advantageous therapeutic ratio by virtue of their low levels of calcaemic activity.