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Recently we have observed that strains of herpesvirus hominis (HVH) could be differentiated antigenically into two types. Type 1 was found to be primarily associated with nongenital infection and transmission, whereas type 2 was primarily associated with genital infection and transmission (1). In order to further characterize the two antigenic types, biologioal markers were sought. Other investigators (2)(3)(4) found that the pock size on embryonated egg choridlantoic membrane (CAM) with certain HVH strains were larger. In addition, various types of histological changes have ;been observed (5-8) in the CAM after infection with different HVH strains. This report presents studies on the relation of pock size on CAM to antigenic type Olf 79 strains of HVH which were isolated from genital and nongenital sites, and suggests the use of this technique as a presumptive test far the differentiation of HVH strains. Histological studies of the CAM infected with serotypes 1 and 2 also are included.Materials and Methods. Virus strains. The source and passage history of the majority of the strains used for this study have been described ( 1 ) . In addition, 4 strains isolated from eye infections, 2 from infections of newborns and a vaginal isolate from an 8-year-old *Supported in part by AI-06157 from National Institutes of Health, USPHS. A. Nahmias is recipient of a Research Career Development Award, IK-AT-18637 from the USPHS.girl are included. The majority of strains were primary isolates with 1 or 2 tissue culture passages, mainly in primary ,rabbit kidney cells. Only 2 strains had prior egg passage.Egg inoculation. Embryonctted eggs 10-1 2 days old were inoculated with 0.1 ml of varying dilutions of virus on the CAM using the false air sac technique (9). At least 2 eggs were inoculated with each virus dilution and the eggs were incubated at 34-35OC for 3-4 days. T o avoid the possible effect of population density on pock size, measurements were made, whenever f easilble, when the number of pocks was between 20 and 100. The membrane was removed onto a plate containing saline. The morphology of the pocks was examined and the &meter of the pocks was measured with the aid of an ocular micrometer mounted to the eyepiece of a binocular dissection microscope. Strains-which did not produce pocks with undiluted virus were tested at least twice.Criteria far "large" and "small" pocks. Up to 2 0 pocks were measured for each strain. Virus strains were divided into those producing "small" or "large" pocks on the h i s of the following criteria: Small-average diameter of pocks counted was less than 0.5 mm and diameter of any one pock did not exceed 1.0 mm (Fig. 1). Large-Average diameter of pocks counted was greater than 0.5 mm and many pocks exceeded 1 mm in diameter (Fig. 2 ) . Pock size measuiremenlts were made on