Comparative studies were made of the reactions of a 'resistant' (Cinco), a susceptible (Planters Jumbo) and a Cinco x Planters Jumbo (Cinbo) cultivar of muskmelon to papaya ringspot virus-watermelon strain (PRSV-W). Neither Cinbo nor Cinco supported levels of virus that were detectable by biotin-ELISA, but PRSV-W was detected in individual cells of the three cultivars. In inoculated leaves of Planters Jumbo most cells were infected, but in both Cinbo and Cinco only 3-7% of cells were infected. When cotyledons of Cinbo were inoculated, mild systemic symptoms were observed, but plants remained symptomless when true leaves were inoculated. Virus was recovered from both cotyledon - inoculated Cinbo grown at 25 or 40-degrees-C and true leaf-inoculated Cinbo, but only when the latter were grown at 25-degrees-C. There was no difference in the percentage of infected cells in these leaves despite the differences in symptom expression. Cinco was highly resistant and only occasionally showed mild systemic symptoms on cotyledon-inoculated plants grown at 25-degrees-C. Unlike those of Cinbo, leaves with symptoms were not infective, but there was no difference in the percentage of infected cells in these leaves compared to those in Cinbo. For both Cinbo and Cinco, several lines of evidence are presented that suggest PRSV-W is restricted to a few cells throughout the plant.