Schizont-infected red blood cells (SI-RBC) from Plasmodium berghei-infected mice contain between 2 and 10 times as much sialic acid as uninfected RBC from the same blood (99–550 μg/1010 RBC versus 33–65 μg/1010 RBC). Total RBC samples from infected animals containing up to 63% ring- and trophozoite-infected cells had identical sialic acid contents to purified RBC samples (of < 3% parasitaemia) from the same blood (52–64 μg/1010 RBC). We conclude that RBC containing immature parasites have the same sialic acid content as uninfected RBC from infected blood and that total cellular sialic acid increases during maturation to the schizont stage. Uninfected RBC from infected blood had 25–50% as much sialic acid as normal mouse RBC (33–65 μg/1010 RBC versus 126 μg/1010 RBC). There were no qualitative changes in RBC sialic acids, all RBC samples having 60–70% N-acetyl neuraminic acid, 30–40% N-acetyl-9-O-acetylneuraminic acid and 5–10% N-gly colylneuraminic acid. The quantitative changes we observed during infection must reflect changes in murine sialoglycoconjugates, as we have shown elsewhere that Plasmodia do not synthesize or contain sialic acids. Since the sialic acid composition of mouse serum glycoconjugates is quite different to that of the RBC fractions studied here, the quantitative data suggest that part of the sialic acids of the uninfected RBC has been transferred to SI-RBC. With higher molar ratios of periodate to substrate than generally used, we were able to radio-isotopically label normal murine sialoglycoproteins on SI-RBC and purified uninfected RBC from infected blood by the periodate/NaB3H4 method. Several new proteins were then tritiated with SI-RBC but these proteins may be intracellular and could even lack sialie acid.