Two viruses, clover yellow mosaic (CYMV) and white clover mosaic (WCMV), were separated from mixed natural infections of various clovers by inoculation of differential hosts. The host reactions of four CYMV and two WCMV isolates were studied in detail. Precipitin tests showed that the two viruses are not related serologically. In cross-absorption tests different host reactions characterizing individual CYMV isolates were reflected in serological interactions; with WCMV isolates the relationship was less clearly defined. A comparison of isolates from different parts of the world by serological methods showed that CYMV is common in western North America, while WCMV occurs in Europe, North America, and New Zealand. Pea mottle and pea wilt viruses may be equated with CYMV and WCMV respectively.