Humulus lupulus (hops) bitter acids, which are well known for their antimicrobial property against Grampositive bacteria have negligible activity against Gramnegative bacteria. The hop acids are, however, antiprotozoal. Ciliated protozoa were more sensitive to hop acids than amoebae. Plasmodia were also sensitive but at a lower level than to the synthetic anti malarial drugs. Beta resin, tetra iso alpha acid and xanthohumol were studied and the latter was found to be particularly potent against the protozoa. Carbon dioxide enhanced the protozoicidal effect of hop acids. New data were also presented on specific antifungal activities. In agreement with the literature hop had very little antifungal property, however a slight coaction was seen between hop and sorbate on R. nigricans. Carbon dioxide had no enhancing effect on the inhibitory activity of hop against fungi as well as E. coli.