Various antimicrobial factors present in human milk were tested for in-vitro antiviral activity against three rhinoviruses (two clinical isolates and rhinovirus 2) and an isolate of cytomegalovirus (CMV) from human milk. These factors included the gangliosides GM1, 2 and 3, sialyl-lactose, chondroitin sulphates A, B and C, prostaglandins E2 and F2alpha, monolaurin, vitamin A and the protein lactoferrin. All were tested for their ability to inhibit growth of the viruses in cell culture. Human milk was also tested for antiviral activity against these viruses. Only vitamin A, monolaurin and lactoferrin inhibited the growth of CMV, whereas both prostaglandins enhanced the growth of this virus at least four-fold. CMV infects infants from milk but, nevertheless, the milk-borne CMV isolate showed no special resistance to any of the antiviral factors tested. None of the compounds inhibited or enhanced the growth of the rhinoviruses. However, human milk decreased the growth of some of the rhinoviruses and specific secretory immunoglobulin A (sIgA) neutralised the virus.