Selected parameters related to work tolerance were measured in 31 adult subjects with hemoglobin (Hb) concentration from 2.5 to 14.0g/100ml. Work tolerance was closely related to Hb concentration (r=0.74) regardless of the adequacy of storage iron level. One male and six females with a mean Hb of 3.5g/100ml (27-55 years old) were studied before and 24hr after transfusing 570ml of whole blood. The mean maximal work load tolerated increased 83% within 24hr after transfusion in these seven subjects. Post-exercise venous blood lactate was markedly lower after transfusion. Work tolerance of these subjects within 24hr after transfusion was the same as in other subjects who had had the equivalent Hb level as the post-transfusion subjects presumably for at least several weeks. These data suggest that the decrement in work performance capacity in iron-deficient and anemic subjects is, in large part, a reflection of the level of anemia rather than other non-Hb related biochemical changes that could accompany prolonged iron deficiency anemia.