This paper examines the elicitor effect of yeast extract (YE) in various concentrations (200–1000 mg/L) on the accumulation of phenolic compounds (PC) in flowering flax (Linum grandiflorum Desf.) cells cultured in vitro and their antiradical activity. It is established that the total PС content and the content of phenylpropanoids increase in the cell culture, especially at high YE concentrations in the medium (500 and 1000 mg/L). The antiradical activity of flax culture extracts remains in most cases at the control level. Therefore, the elicitation of flowering flax in vitro cultures by YE activates the PC biosynthesis resulting in the accumulation of these secondary metabolites, while the antiradical activity of cell culture extracts does not decrease compared to the control level.