This paper describes two consecutive studies: a volume study and an efficacy study. The volume study determined the appropriated volume of Medical Grade Silicone Rubber (MSR) needed to achieve complete occlusion of the vas deferens. This was done by in-vitro testing of 130 human vas specimens containing plugs of MSR formed in vivo. The volume of MSR needed to occlude the vas was 0.1531 +/- 0.0059 ml injected by five to six turns of the applicator handwheel. There was a correlation between MSR volume, weight, and the number of turns of the applicator handwheel. An influence of body height upon the volume and weight of the MSR was also observed. An oval shaped clamp (15 mm long) was more effective in producing secure vas occlusion than was a round clamp (10 mm long). The mean outer diameter of the vas specimens was 1.80 +/0 0.15 mm, and the mean maximum dilated inner diameter was 0.93 +/- 0.11 mm. The efficacy study was a clinical trial to compare MSR was occlusion (using an oval 15 mm clamp and the appropriate volume derived from the volume study, n = 58) with no-scalpel vasectomy as the standard procedure (n = 64). The azoospermia rate following MSR vas occlusion was not significantly different from that following no-scalpel vasectomy, and was achieved in 3-6 months.