Cowpea chlorotic mottle virus (CCMV) and cowpea protoplasts were used to study initial interactions between virus and protoplast. Protoplasts and virus were incubated under varying conditions of temperature, pH, ionic strength, and the presence of added compounds. Both the amount of 35S-labeled virus bound to protoplasts and the percentage of infected cells were determined. At 0 and 25 degrees the amount of virus associated with protoplasts increased with the amount of virus added. With inoculum of 25 x 10(6) virus particles per protoplast, 4 x 10(3) and 14 x 10(3) particles per protoplast were bound at 0 and 25 degrees, respectively. In the presence of polyethylene glycol, 85 x 10(3) associated particles per protoplast were bound at both temperatures and ca. 50% of the protoplasts became infected. No infection occurred in the absence of PEG. Variation of pH or ionic strength in the absence of PEG caused little to no change in binding and no infection. In the presence of PEG, increase of pH resulted in lower binding, but infectivity was not affected. Increasing ionic strength, however, increased both binding and infectivity. The presence of unlabeled CCMV, tobacco mosaic virus coat protein, bovine serum albumin, and polycations during inoculation in the absence of PEG decreased the amount of bound CCMV. In contrast, CCMV coat protein, which has a positively charged N-terminal arm, increased binding. In the presence of PEG the effects were similar, although larger amounts of virus were bound. The percentage of infection was reduced by all additives to 5-25%. Addition of ammonium chloride, which inhibits endocytotic virus uptake in animal cells, during inoculation as well as in culture media, did not reduce infectivity. These data do not support a specific receptor-mediated endocytotic uptake of virus but favor a nonspecific mechanism of entry, possibly through membrane lesions. Observations in the electron microscope support the latter mechanism.