Cell membrane function during hybridization was assessed by examining the accumulation of four representative amino acids and a sugar in mouse-human hybrid cells, parental mouse cells and human cells. Qualitatively, accumulation in the hybrids was found to resemble the accumulation in parental mouse more than the human cells. In particular, the steady-state uptake of leucine and glycine in hybrids was similar to that in parental mouse cells and different than that observed in the human cells. The findings suggest that hybrids retain transport properties of mouse rather than human cells. These results are in keeping with our previous observation that the human mouse hybrids conserve most of the mouse chromosomes and the membrane proteins of the hybrids are very likely of mouse parental origin.