This study investigated the trypsin digestion and serum inhibition in articular cartilage monitored using high-frequency ultrasound system in situ. Three normal bovine patellae were obtained from a local butcher shop. Four full-thickness cartilage-bone specimens were prepared from the lower medial side of each patella and divided into 4 groups: Group A, B, C and D. Group A was treated as control. The samples from the other three groups were digested by trypsin for 40 minutes and then were immersed for 280 additional minutes in trypsin, physiological saline, or fetal bovine serum (FBS), respectively. It was measured in the M-mode ultrasound images that 30% to 54% of the full-thickness cartilage tissue for all the samples was digested by the 40-minute trypsin digestion. The continuous trypsin digestion depleted almost all proteoglycan (PG) content of the samples of Group B. For Group C, the residual enzyme digestion in saline generated a digestion extension only slightly less than that of Group B. The enzyme digestion in the samples of Group D was relatively quickly (approximately 1 hour) stopped by FBS. The final ultrasound data were compared to histology for assessing the PG depletion. Our findings provide important information of enzyme digestion and inhibition inside articular cartilage. Ultrasound represents a useful visual tool to evaluate the dynamics of models of OA in cartilage specimens in vitro.