Poly-l-lactic acid (PLLA), a synthetic, biocompatible, biodegradable polymer, has been safely used in several clinical applications in recent decades. Typically, SculptraTM, the commercially injectable PLLA in the form of microparticles, has been used as facial volumizer in the treatment of lipoatrophy in HIV patients. It also has various applications in tissue engineering by improving cell proliferation and adhesion. Sculptra™ can be categorised as a stimulatory filler as it stimulates the synthesis and deposition of fibrous tissue and collagen. Collagen is one of the most significant components of the extracellular matrix and beneficial for the normal physiology. It is also the structural component of a human body. In most of the studies, the effect of Sculptra on collagen synthesis was investigated in vivo and the majority of the data were from clinical and histological reports. There is only one study reporting this effect in vitro using fibroblasts. Here, we investigated whether PLLA in the form of nanoparticles can provide the same effect on collagen synthesis in fibroblasts as Sculptra. We surprisingly found that there was no stimulation of collagen in fibroblasts alone, whereas the co-cultures of fibroblast and macrophage had shown collagen stimulation by PLLA nanoparticles. It is also confirmed that collagen synthesis was caused by fibroblasts but not macrophages. Although further study needs to be conducted to evaluate its mechanism, our findings showed that choosing an appropriate method is essential for investigating the effect of PLLA or other biomaterials on collagen synthesis by fibroblasts in vitro.