The insulin-like growth factor II (IGF-II) and glycoprotein lysosomal enzymes containing mannose 6-phosphate (M6P) bind with high affinity to two separate sites on the same receptor molecule (Morgan et al. Nature 329:301). The addition of free M6P significantly increases the affinity of some preparations of the M6P/IGF-II receptor (M6P/IGF-II-R) for IGF-II. We conducted this study to test the hypothesis that this effect is the result of displacement of M6P-related ligands that inhibit IGF-II binding. First we found that although M6P caused a 66% increase in the binding of IGF-II to microsomes prepared from IM9 cells, it had no effect, under identical conditions, on binding to receptor on the surface of intact cells. Secondly, extensive washing of rat liver microsomes in the presence of 3 mM M6P, followed by removal of the M6P by further washing, abolished the effect by raising binding to levels seen in the presence of M6P. M6P, then, had no additional effect. Finally, when IGF-II-affinity purified receptor was repurified by ultracentrifugation on a sucrose gradient, binding to the pure receptor peak was not affected by M6P. We conclude that there is no intrinsic positive cooperativity between free M6P and the IGF-II-binding site of the M6P/IGF-II-R. The reported M6P-induced increase in IGF-II binding appears to be due to the displacement of contaminating inhibitory endogenous ligands.