A preparation having high anti-rachitic value for chickens is obtained by irradiation of the unsaponifiable or sterol fraction of ducks' eggs.  The fraction may be further purified before irradiation to give a concentrated pro-vitamin.  According to examples, materials obtained as described below were subjected to irradiation by a quartz mercury lamp for 30 minutes in an evacuated quartz vessel, the light from the lamp being filtered through a solution of xylene in alcohol and the intensity being so regulated that after the irradiation about 40 per cent of the absorbing sterol was converted.  Example 1: acetone extract of the yolks of Dutch ducks' eggs was saponified with alcoholic KOH, the aqueous soap solution extracted with ether, and the extract recrystallized from alcohol.  Example 2: acetone extract of Chinese ducks' egg yolk, was saponified in usual manner, and calcium soap precipitated by addition of CaCl2, the precipitate separated and extracted with acetone.  The acetone extract was recrystallized from alcohol and the sterol mixture acetylated and recrystallized from alcohol.  The sterol acetate was further purified by passing a solution of the acetate in benzene and petrol ether into an absorption column containing Al2O3 and then pouring in a further quantity of benzene and petrol ether, the filtrate being collected in fractions of which the later contained the higher proportion of pro-vitamin acetate.  After further similar purification and recrystallization the pro-vitamin acetate was saponified and recrystallized from methyl alcohol.  The products obtained according to the above examples were dissolved in ether for irradiation.ALSO:A preparation having high anti-rachitic value for chickens is obtained by irradiation of the unsaponifiable or sterol fraction of ducks' eggs.  The fraction may be further purified before irradiation to give a concentrated pro-vitamin.  According to examples, materials obtained as described below were subjected to irradiation by a quartz mercury lamp for 30 minutes in an evacuated quartz vessel, the light from the lamp being filtered through a solution of xylene in alcohol and the intensity being so regulated that after the irradiation about 40 per cent of the absorbing sterol was converted.  Example 1. Acetone extract of the yolks of Dutch ducks' eggs was saponified with alcoholic KOH, the aqueous soap solution extracted with ether, and the extract recrystallized from alcohol.  Example 2.  Acetone extract of Chinese ducks' egg yolk, was saponified in usual manner, and calcium soap precipitated by addition of CaCl2, the precipitate separated and extracted with acetone.  The acetone extract was recrystallized from alcohol and the sterol mixture acetylated and recrystallized from alcohol.  The sterol acetate was further purified by passing a solution of the acetate in benzene and petrol ether into an absorption column containing Cl2O3 and then pouring in a further quantity of benzene and petrol ether, the filtrate being collected in fractions of which the latter contained the higher proportion of pro-vitamin acetate.  After further similar purification and recrystallization, the pro-vitamin acetate was saponified and recrystallized from methyl alcohol.  The products obtained according to the above examples were dissolved in ether for irradiation.