C3 activity is required to neutralize SV5 and MuV. A) Total hemolytic complement activity of NHS, C3-depleted serum, and C3-depleted serum reconstituted with C3 was determined as described in Materials and methods. B) and C) 100 PFU of SV5 (panel B) or MuV (panel C) were incubated with PBS (hatched bars), NHS (black bars), with a 1:25 dilution of C3-depleted serum (striped bars), or 1:25 dilution of C3-depleted serum reconstituted with C3 (gray bars). Remaining infectious titers were determined by plaque assays. Results are the average of six experiments, with error bars representing standard deviations. Number signs denote p < 0.001 comparing C3-depleted samples to C3 reconstituted samples. D) C3 deposition on SV5 and MuV. Sucrose gradient-purified virus was left untreated or treated with 1:25 diluted NHS or C3-depleted serum, and complement C3 deposition on the particles was detected using anti-C3 antibody followed by 12 nm colloidal gold goat anti-mouse antibody. Samples were analyzed by EM at a magnification of 55,000 × (bar represents 0.05 μm).