The surface membrane proteins of red blood cells from normal, hyperbled or acetylphenylhydrazine-treated BALB/c mice and NZB mice of different ages were labelled by lactoperoxidase-catalyzed radioiodination. Sialoglyoproteins were labelled by periodate/NaB3H4 or galactose oxidase +/- neuraminidase/Na3H4 treatments. 2. Anaemia produced several changes in radioiodinated proteins. 3. Sialoglycoprotein radiolabelling was unchanged, even with over 90% reticulocytosis. 4. Decreased periodate/NaB3H4-dependent labelling of red blood cells from Plasmodium berghei-infected BALB/c mice (Howard et al., 1980; Howard & Day, 1981) cannot therefore be due to anaemia per se, but must be related more specifically with infection.