The aim of this study was to verify the in vitro influence of various dilutions of a viscoelastic substance containing 2% hydroxy-propyl-methylcellulose (HPMC) on the viability of some microbial strains representative of the normal flora of the external eye. Pure reference strain cultures of <i>Candida albicans, Pseudomonas aeruginosa, Propionibacterium acnes, Staphylococcus aureus, Staphylococcus epidermidis</i> and a fresh clinical isolate of <i>Proteus mirabilis</i> were selected for this study. Serial twofold dilutions of 2% HPMC, prepared so as to obtain a final concentration ranging from 50 to 0.78% of the product in sterile saline solution (0.85% NaCl), were taken out with a pipette that delivered 1.0 ml per tube. One hundred microliters of the abovementioned microbial inocula, used for the evaluation of the positive control of the test organism, were dispensed into each tube. After 24 h of incubation, 100 μl of samples were taken from each tube and plated into the specific medium for the growth of the test organism. After 24–48 h of incubation, these agar plates were examined and the colony-forming-unit count of each test organism was compared to the corresponding total colony count, acting as a positive control, in order to determine the quantitative variation of the test organism grown in the presence of the viscoelastic compounds. <i>C. albicans</i> and <i>P. aeruginosa</i> showed a statistically significant increase in growth with HPMC dilutions varying from 1:2 to 1:16. <i>P. acnes</i> and <i>P. mirabilis</i> growth was significantly reduced by all dilutions except for the 1:128 one. <i>S. epidermidis</i> growth was also significantly reduced in the presence of HPMC dilutions varying from 1:2 to 1:64. <i>S. aureus</i> growth was not significantly influenced. The viability of <i>P. aeruginosa</i> in HPMC dilutions needs to be carefully considered because of the ability of this organism to induce endophthalmitis, and the possibility that during cataract surgery, a small amount of HPMC may be left in the eye, trapped behind the intraocular lens optic.