We have shown previously that microneme secretion is inhibited below 258C (Wan et al., 1996); therefore, we reasoned that, if invasion relies on microneme proteins, it should exhibit the same temperature dependence. To examine the relationship between invasion and microneme secretion, we monitored parasite invasion of host cells by enzymatic detection of the heterologous reporter b-Gal expressed by a transgenic parasite strain (2F). This assay provides a highly sensitive and convenient means of quantifying parasite numbers (Dobrowolski and Sibley, 1996). In parallel, microneme secretion was detected by Western blot analysis of culture supernatants. Consistent with the hypothesis that invasion depends on microneme proteins, a direct correlation between basal secretion of MIC2 into the culture supernatant and parasite invasion of HFF monolayers was observed between 258C and 408C (Fig. 1). Similar results were obtained for MIC1 and MIC4 using monoclonal antibodies speci®c for these proteins (data not shown). In contrast, secretion of the dense granule protein GRA1 into the supernatant was