The calcium binding L1 protein was found to inhibit growth of blood culture isolates of Candida spp and cerebrospinal fluid isolates of Cryptococcus neoformans. Minimum inhibitory concentrations (MIC) were 4-128 mg/l, and concentrations 2-4 times the MIC were fungicidal. Blood culture isolates of Escherichia coli, Klebsiella spp, Staphylococcus aureus, and Staphylococcus epidermidis had MIC values of 64-256 mg/l. Antibacterial activity was strongly influenced by the nature of the culture medium. In view of the biological activity of L1, the name calprotectin is proposed to describe this antimicrobial protein with calcium binding properties.