[en] Column densities of six interstellar species including CH, CH⁺ and CN in the direction of the O-type supergiant HD 154368 are presented. Using the strong CN lines, a value for the excitation temperature at lambda = 2.64 mm is obtained, namely T₁₀ = 2.8 +- 0.4 K. A secure upper limit is T₁₀ < 3.5 +- 0.6 K. This result is similar to values found in other directions and is consistent with a microwave background origin. Upper limits to Tsub(ex) at other wavelengths are derived using all three molecular species. Observations of HD 154368 at higher resolution should allow determination of Tsub(ex) at 1.32 mm. (author)