The effect of supplementing a regular diet with riboflavin or a combination of riboflavin and ascorbic acid on haematological indices was studied in 27 young Nigerian adults. Vitamin supplementation produced a significant increase (p < 0.001) in haemoglobin concentration, haematocrit level and erythrocyte count. Both males and females responded similarly to the supplementation. The effect of riboflavin was similar to the combined effect of riboflavin and ascorbic acid. A significant association suggestive of enhanced erythropoiesis existed between Hb concentration and erythrocyte count in vitamin‐supplemented subjects (r = 0.9722, p < 0.002). The withdrawal of vitamin(s) supplements significantly diminished Hb concentration, haematocrit level and erythrocyte count to values similar to placebo. The data further showed that, even in malarial infection, Hb concentration, Hct level and erythrocyte count were maintained if high vitamin status was established through supplementation.