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Background: Subcutaneous grafting/implantation of neonatal testis tissue/cells from diverse donor species into recipient mice can be used as an in vivo model to study testis development, spermatogenesis, and steroidogenesis. Ultrasound biomicroscopy (UBM) allows obtaining high definition cross‐sectional images of tissues at microscopic resolutions. Objectives: The present study was designed to (a) validate the use of UBM for non‐invasive monitoring of grafts/implants overtime and to (b) correlate UBM findings with the morphological attributes of recovered grafts/implants. Materials and methods: Testis tissue fragments (~14 mm3, each) and cell aggregates (100 × 106 cells, each) obtained from 1‐week‐old donor piglets (n = 30) were grafted/implanted under the back skin of immunodeficient mice (n = 6) in eight analogous sites per mouse. Three‐dimensional transcutaneous Doppler UBM was performed, and a randomly selected graft and its corresponding implant were recovered at 2, 4, 6, and 8 weeks. Results: Graft/implant weight (P =.04) and physical height (P =.03) increased overtime. The dynamics of physical length and volume increases over time differed between tissue grafts and cell implants (P =.02 and 0.01 for sample type*time interactions, respectively). UBM‐estimated volume was correlated with the post‐recovery weight and volume of the grafts/implants (r = 0.98 and r = 0.99, respectively; P <.001). Pre‐ and post‐recovery length and height of the grafts/implants were positively and strongly correlated (r = 0.50, P =.01; r = 0.70, P =.001) and so were the areas covered by cordal, non‐cordal, or fluid‐filled cavities between UBM and histology (r = 0.87, P <.001). Discussion and conclusion: UBM findings correlated with physical attributes of the grafts/implants, validating its use as a non‐invasive high‐fidelity tool to quantify the developmental changes in ectopic testis tissue grafts and cell implants, potentially leading to a reduction in the number of recipient mice needed for similar experiments. [ABSTRACT FROM AUTHOR]