Platelet aggregation to four different agonists was measured in platelet-rich plasma from 17 patients with nutritional vitamin B12deficiency; 10 of these patients were also iron deficient. Impaired aggregation to ADP, collagen, epinephrine and ristocetin was found for 4, 8, 10 and 6 patients, respectively. Bleeding manifestations were seen in 5 patients, one belonging to the vitamin B12deficiency group and 4 to the combined deficiency group. Three of these 5 patients had normal platelet aggregation. These results show that impaired platelet aggregation is common in nutritional vitamin B12deficiency but that this impairment does not reliably correlate with a clinical bleeding tendency.