A technique to recognize and quantitate oncornaviruses using perforated pointed BEEM capsules has been developed in our laboratory. Virus samples that presented problems in counting, or those which could not be evaluated at all by negative staining, could be clearly defined and counted using the thin-sectioning BEEM capsule technique. Perforation of the BEEM capsule allowed rapid infiltration of reagents into the tip of the virus pellet and made further manipulation and orientation unnecessary. The sensitivity of this technique, determined by making serial dilutions of viral concentrates, allows observation of as few as 5 times 10(5) virus particles per ml. Precision in counting by this technique varied only +/- 0.3 log in repeat aliquots of identical concentrated virus samples quantitated, making this a highly useful and reliable system.