Three synthetic 6,19-carbon bridged steroids: 3beta,20beta-diacetyloxy-5alpha-chloro-19a(R)-hydroxy-6,19-methanopregnane, 3beta,20beta-diacetyloxy-5alpha-chloro-6,19-methanopregnane, 6,19-methanopregn-4-ene-3,20-dione and four synthetic precursors: 3beta,20beta-diacetyloxy-19-hydroxypregn-5-ene, 3beta,20beta-diacetyloxy-pregn-5-en-19-al, 3beta,20beta-diacetyloxy-19(E)-(methoxymethylidene)-pregn-5-ene and 20beta-acetyloxy-3beta-hydroxy-19(E)-(methoxymethylidene)-pregn-5-ene were tested against herpes virus replication in cell cultures. Several compounds were cytotoxic for stationary cells. Antiviral studies performed with all compounds against HSV-1 indicated a dose-dependent virus susceptibility with selectivity indexes (SI) values in the range 1.7-183.2. Selected compounds were also tested against HSV-2 and the SI values obtained were in the range of 31-273. Attempts to reveal the step of virus multiplication affected by pregnanes were performed with one compound. HSV-1 virus incubation with the compound did not alter the ability of virus particles to infect cells; moreover, neither virus adsorption nor penetration appeared to be affected. The drug must be present during at least the first 7 h of the virus cycle to inhibit more than 90% of virus production. All these results suggest that these novel molecules interfere with an intracellular step of virus multiplication, thus behaving like true antivirals.