The total entry of ammonium ions into Sp2/0-Ag14 myeloma cells and hybridoma cells consists of a saturable and a non-saturable component. The plasma membrane Na(+)K(+)2Cl(-)-cotransporter was identified as the saturable ammonium ion transporter in both cell lines, and the non-saturable entry was due to simple diffusion of ammonium ions. The theoretical maximum transport rate via the Na(+)K(+)2Cl(-)-cotransporter was identical in the two cell lines, but the ammonium ion diffusion rate was considerably higher in the hybridoma cells. We speculate that this is an effect of different membrane properties caused by dissimilar expression of tumour characteristics.