The absolute density of SD radicals in a supersonic jet has been measured down to $(1.1pm0.1)times10^5$ cm$^{-3}$ in a modestly specified apparatus that uses a cross-correlated combination of cavity ring-down and laser-induced fluorescence detection. Such a density corresponds to $215pm21$ molecules in the probe volume at any given time. The minimum detectable absorption coefficient was quantum-noise-limited and measured to be $(7.9pm0.6)times10^{-11}$ cm$^{-1}$, in 200 s of acquisition time, corresponding to a noise-equivalent absorption sensitivity for the apparatus of $(1.6pm0.1)times10^{-9}$ cm$^{-1}$ Hz$^{-1/2}$.
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