We sought to examine phosphorothioate oligodeoxynucleotide (PS oligo) non-G-quartet, nonsequence specific effects on smooth muscle cell (SMC) adhesion by using S-dC28, a 28-mer cytidine homopolymer that lacks contiguous guanosine residues. Human aortic SMC were incubated with vehicle or various doses of S-dC28, and the number of SMC adhered to noncoated plates was determined using a Coulter Counter. S-dC28 significantly inhibited SMC adhesion. SMC adhesion dramatically improved with S-dC28 in fibronectin-coated plates. When laminin-coated plates were used, the inhibition of SMC adhesion by S-dC28 was completely reversed. Replacement of serum-free medium with 5% fetal bovine serum medium for SMC cultured on non-coated plates also greatly attenuated inhibition of adhesion by S-dC28. Human SMC TPA, UPA, and PAI-1 antigen levels were determined by ELISAs. PDGF significantly induced SMC TPA antigen production measured by an ELISA. Coincubation of PDGF with S-dC28 significantly attenuated SMC TPA antigen levels. SMC UPA antigen levels after coincubation with PDGF and S-dC28 were significantly greater than the values observed in SMC incubated in medium containing PDGF alone. SMC PAI-1 antigen levels were not altered by the addition of S-dC28. We conclude that S-dC28 inhibits human SMC adhesion and that this inhibition can be diminished by fibronectin or laminin coating of culture plates or by the presence of serum in the culture medium. Furthermore, S-dC28 attenuates SMC TPA production, thereby inhibiting SMC migration, whereas S-dC28 augments SMC UPA production, thus diminishing SMC cellular adhesion.