Cells of the marine sponge Microciona prolifera were placed in calcium-magnesium-free artificial sea water and reaggregated with Ca 2+ (CaCl 2, 12 or 24 mM). The oxidant stressors hydrogen peroxide (H 2O 2) and bleach (NaClO) exerted significant effects on Ca 2+-induced aggregation, and these effects were markedly altered by the pH of the suspending fluid. At pH 7.0–7.1, 10 min in 100 μM H 2O 2 or 20 mM bleach significantly inhibited calcium-induced aggregation, whereas at pH 6.5–6.6, these concentrations potentiated aggregation. The enzyme catalase partially or completely reversed the effects of both H 2O 2 and bleach, suggesting that the effect of bleach was mediated by the formation of H 2O 2, possibly through lipid peroxidation. Attempts to study intracellular calcium fluxes were foiled by an apparent lack of the intracellular esterase necessary to cleave the acetoxymethyl ester from indicator dyes (fura-2, fluo-3). A compound (1-tetradecylamine), known to release calcium from intracellular stores of mammalian cells, induced significant aggregation at pH 6.5 but had minimal effects at pH 7.0. The results of this study suggest that moderate pH shifts markedly affect the aggregation response of these sponge cells to oxidant stressors, possibly by affecting cell permeability and/or the release of intracellular calcium. Comparisons with mammalian cells and other marine species are discussed.