The authors investigated the resonant M1-E1 two-photon transition (6S1/2to 7S1/2to 15P3/2) in caesium using counterpropagating single-mode laser beams in a thermionic diode. They measured circular polarization correlations, i.e. the dependence of the two-photon transition rate on the relative helicities of the two circularly polarized beams. The results depend strongly on the intensity of the laser driving the 7S to 15P transition and are in good agreement with theoretical predictions from a rate equation treatment taking collisional population redistributions among the 15p32/states into account. The implications of the results for a field-free parity violation experiment are discussed. © 1991 IOP Publishing Ltd.