The possibility of observable recombination lines below 200 MHz is shown to be greatest for the case of negative absorption in low-density gas in front of strong radio sources. The amplification of background radiation can conceivably exceed 100 per cent, especially when the electron temperature is low; on the other hand if the temperature is sufficiently high, dielectronic recombination may give rise to detectable recombination lines from elements other than hydrogen. The optimum frequencies are in the range 50–200 MHz for the interstellar medium, and 1–100 MHz for such intergalactic matter as may exist. The effects of external radiation fields, thermal and nonthermal, are explored quantitatively and the relevantb_n-factors computed for a 1000-level atom; it is found that even the general galactic radiation field will significantly alter the atomic level populations. Observations of these masing low-frequency recombination lines would be useful in sorting out the hot and cold components of the interstellar medium, and in investigating their properties.