PURPOSE: An inexpensive growth agent, consisting of an N-acyllactam compound having a specific structural formula, synthesizable, and usable for bacteria of the genus Bifidobacterium, e.g. Bifidobacterium bifidus, useful for medicines, foods, etc.

CONSTITUTION: A growth agent consisting of an N-acyllactam compound expressed by formula I [R1 is H, CH3-, CH3-CO- or formula II or III; R is -CH2-, -(CH2)2- or -CH=CH-; n is 3W5], e.g. 1-[3-(4-hydroxyphenyl)propanoyl]-2- piperidone. For example, the above-mentioned compound can be obtained by the direct reaction of a lactam with an acid chloride.