The aim of the paper was to determine benz(a)-pyrene in the preparations containing humates and study the benz(a)pyrene biological potency for the agricultural plants. The research methodology included the determination of the dependencies in the system “substance concentration (dose)—effect on the plant”. Concentrations of benz(a)pyrene in 12 samples of the humates preparations and fertilizers based on their trademarks “Irkutsk humates”, obtained from brown coal, varied in the range from 0.3 to 50 mcg/kg, which creates no soil contamination in conditions of the use of preparations. Between contents of benz(a)pyrene and humates there is a correlation (r_xy = 0.95; α = 0.05). It is ascertained that the effects of stimulation and/or inhibition of the growth and yield of agricultural plants depend on the concentration of benz(a)pyrene and the method of plant processing. Optimal concentrations of benz(a)pyrene were 150-200 ng/dm³ for preplant way of processing of potato tubers, 3-10 ng/dm³—for top dressing (spraying) and 0.1-0.3 ng/dm³—for dressing under the roots (hydroponic). The obtained results allowed us to offer one of the possible mechanisms of biological potency of humates as the plant growth stimulants, and also a way testing of the preparations by screening of their benz(a)pyrene content.