Increasing literature suggests that cell adhesion molecule alpha4beta1 integrin plays a pivotal role in autoimmune diseases and cancer development. Noninvasive visualization of alpha4beta1 integrin in vivo will facilitate the understanding of its involvement in disease progression and development of targeted therapies. Due to the lack of high-affinity targeting ligands, molecular imaging of alpha4beta1 integrin is much less explored than that of alphavbeta3 and alphavbeta5 integrins. We have recently reported using the one bead-one compound combinatorial library method to identify a peptidomimetic, LLP2A, that preferentially binds to activated alpha4beta1 integrin. Here, we described the use of LLP2A-Cy5.5 conjugate as an in vivo optical imaging probe in a human lymphoma xenograft model. This univalent LLP2A-Cy5.5 conjugate retained the binding activity and specificity to alpha4beta1 integrin as shown by cell binding assays using alpha4beta1-positive Molt-4 T-leukemia cells. The subcutaneous Molt-4 tumor was clearly visualized from 1 to 24 h after tail vein injection of the conjugate. Direct imaging and confocal microscopic examination of excised tumors and organs confirmed the accumulation of LLP2A in tumors and revealed very little or no uptake in normal organs except for lymph nodes. Kidney uptake was high when the whole organ was scanned but it was negative when examined microscopically, suggesting that LLP2A bound to the renal tubules loosely. Tumor uptake of LLP2A-Cy5.5 conjugate was blocked by excess unlabeled LLP2A. This study showed that the combinatorial chemical library-derived peptidomimetic LLP2A can be easily developed into an optical imaging probe for noninvasively monitoring of activated alpha4beta1 integrin in vivo.