Abstract. In the paper, the authors find necessary and sufficient conditions for a difference between the exponential function αe β/t , α, β > 0, and the trigamma function ψ (t) to be completely monotonic on (0, ∞). While proving the complete monotonicity, the authors discover some properties related to the first order modified Bessel function of the first kind I1, including inequalities, monotonicity, unimodality, and convexity.Mathematics Subject Classification (2010). Primary 26A48, 33C10; Secondary 26A51, 33B10, 33B15, 44A10.