Cellular effects of low-power laser irradiation on Jurkat E6.1 T-lymphocyte leukemia cells were examined in vitro at 635nm (visible) and 780nm (near infrared) wavelengths. The cells were exposed to three energy densities (6.116, 9.174 and 12.232 J/cm2), and then examined post-irradiation after three incubation time periods (24, 48 and 72 hours). The examination involved assessments of cell proliferation and viability, cell cycle progression, apoptosis, real-time PCR analysis of p21 gene and expression of two heat shock proteins; HSP70 and HSP90.