Cylindrical inclusion proteins (CIPs), amorphous inclusion proteins (AIPs), and virus particles were partially purified from Cucurbita maxima leaves infected with zucchini yellow mosaic virus (ZYMV), watermelon mosaic virus 2 (WMV2), and the watermelon mosaic virus 1 strain of papaya ringspot virus (PRSV-W). Antisera to these individual antigens, except AIPs of ZYMV and WMV2, were prepared and used to determine with electroblot-ELISA the serological differentiation indices (SDIs) between homologous and heterologous antigens. The average SDIs between ZYMV and WMV2 CIPs, between ZYMV and PRSV-W CIPs, and between WMV2 and PRSV-W CIPs were 1.5, 3.5, and 2.5, respectively. The SDIs between ZYMV and PRSV-W AIPs and between WMV2 and PRSV-W AIPs were 5 and 3, respectively. The average SDIs between ZYMV and WMV2 capsid proteins (CPs), between ZYMV and PRSV-W CPs, and between WMV2 and PRSV-W CPs were 3, 7, and 8, respectively. These data suggest that the serological relationship between ZYMV and WMV2 is much closer than that between PRSV-W and ZYMV or between PRSV-W and WMV2 and that antigenic determinants of CIP and AIP were conserved more than those of CP among the three potyviruses.