The biochemical cause of human retinitis pigmentosa (RP) is unknown, although in kittens it is related to taurine deficiency. The concentrations of free amino acids were therefore determined in the plasma and platelets of 16 patients with RP and 27 controls. No statistically significant change was found in the plasma, but the concentration of taurine and aspartic acid of platelets calculated per number of platelets were significantly (p < 0.05) lower in RP patients than in controls. This finding correlates with the decreased uptake of taurine by platelets of RP patients and may be connected with the pathogenesis of this disease.