The initial rates of aminoacid influx to erythro cytes from nine patients on maintenence haemodialysis and from nine normal controls have been studied using radiolabelled L-serine, glycine, L-tryptophan and L-leucine. L-serine uptake had an Na-dependent component fitted by Michaelis-Menten kinetics (ASC system) and a non-saturable Na-independent component. In erythrocytes from haemodialysis patients L-serine uptake via the ASC system had a mean (SEM) Vmax of 233 (33) micromol/1 cells per h, compared to a mean Vmax in controls of 353 (21) micromol/l cells per h. The mean Km for L-serine uptake was 105 (11) microM in haemodialysis erythrocytes and 160 (12) microM in controls. These reductions in Vmax and Km for ASC in haemodialysis erythrocytes were significant (2P < 0.02). Glycine uptake had a Michaelis-Menten-type chloride-dependent component (gly system), and a chloride-independent non-saturable component. The mean Km for glycine influx via the gly system was 26.3 (2.3) microM in haemodialysis erythrocytes, and 34 (3.1) microM in normals (2P < 0.02). The Vmax for the uptake of glycine by the gly system did not differ significantly between haemodialysis and control erythrocytes. The initial influx rates of L-tryptophan via the T system and of L-leucine via the L system did not vary between control and haemodialysis erythrocytes. It is suggested that abnormalities of the ASC and gly membrane transport systems may contribute to the altered interorgan transport of aminoacids found in renal disease.