Boar and ram spermatozoa were incubated in Tyrode's medium in the presence or absence of bicarbonate/CO2, a component believed essential for capacitation. At intervals, samples were stained with a range of FITC-lectins to detect changes in surface glycoconjugates, using a rapid staining technique to avoid problems of lectin toxicity. The samples were then analysed directly by flow cytometry, using propidium iodide to distinguish dead cells. In the presence of bicarbonate, a live subpopulation of spermatozoa developed, which in both animal species showed higher binding affinities towards Phaseolus Vulgaris Agglutinin (PHA-E), Sophora Japonica Agglutinin (SJA), and Soybean Agglutinin (SBA), and lower binding affinity towards Erythrina Cristagalli Lectin (ECL). In boar samples, the modified subpopulation reached a maximum after 3 hr incubation, whereas in ram samples it maximized after 1.5 hr. No changes were seen when spermatozoa were incubated in bicarbonate-free medium. The bicarbonate-induced changes in lectin binding were not due to the onset of acrosome reactions, because spermatozoa induced to undergo acrosome reactions with the ionophore A23187 displayed very different lectin-binding patterns. Tested on boar spermatozoa, seminal plasma not only inhibited but reversed the bicarbonate-induced development of the modified subpopulation. EGTA also inhibited development of boar sperm subpopulations; excess Ca2+ was unable to overcome this inhibition, suggesting that multivalent metal ions might be involved in bicarbonate's action. We conclude that bicarbonate causes a loss of surface coating material with affinity for ECL and an unmasking of binding sites for SBA, SJA and PHA-E. A modified subpopulation of live spermatozoa is thereby established, which appears to maximize at a rate in accord with reported capacitation times.