Linear growth rate of the RTI for a partially ionized plasma as a function of the ambipolar diffusion coefficient η_A2 with some values of the neutral fraction ξ_n2 for which the values of η_A2 are obtained and also included in the plot as large points. The values ρ₂/ρ₁ = 100, β = 0.1, k = 10^-7 m^-1, θ = 40°, and ξ_n1 = 10^-6 have been used (so c_crit = 47 km s^-1). The upper panel corresponds to a classically unstable configuration, the middle panel to a marginally stable configuration, and the lower panel to a classically stable configuration. The dotted line in the upper panel corresponds to the MHD limit (in the middle and lower panel the classical limit is zero).