Erythrocyte lithium transport mechanisms--lithium-sodium countertransport (LSC), lithium-potassium cotransport (LPC) and passive lithium efflux (PLE)--were measured in 46 patients with bipolar affective disorder on prophylactic lithium therapy and in 20 healthy control subjects. Maximal velocity of LSC measured at saturating intracellular lithium concentration was lower in the patients than in the controls; this may concur with previous reports on possible links between impaired activity of LSC and bipolar affective illness. When measured at therapeutic lithium concentration, LSC was 4 times lower and Km for LSC was 5 times higher in lithium-treated affective patients than in control subjects. The in vivo erythrocyte:plasma lithium ratio was inversely correlated with LSC in lithium-treated patients; higher ratios were found in females than in males. No differences were found between affective patients and control subjects in other erythrocyte lithium transport measurements. The values for lithium transport were not related to age, duration of lithium therapy, concomitant neuroleptic treatment, hypertension or obesity. Lower activity of LSC was found in patients with lithium-induced thyroid enlargement than in the other patients. The results obtained are discussed in the light of contemporary findings concerning erythrocyte lithium transport mechanisms in affective disorders and other conditions.