Freshly isolated rabbit proximal tubules (PT), confluent primary rabbit proximal tubule cultures (PTC) and LLC-PK1 cells were characterised. Brushborder enzyme activities were lower in PTC than in LLC-PK1: ratios were 0.026 for alkaline phosphatase (AP), 0.458 for alanine aminopeptidase (AAP) and 0.514 for gamma-glutamyl transpeptidase (GGT). PT/PTC ratios were 79.7 for AP, 7.96 for AAP and 3.45 for GGT. Specific activities of hexokinase (HK) and lactate dehydrogenase (LDH) were high in cultured cells as compared to PT: PT/PTC ratios were 0.063 and 0.033, while PTC/LLC-PK1 ratios were 0.406 and 1.19 for HK and LDH respectively. PTC/LLC-PK1 ratios were 2.21 forNa/K ATPase, 2.07 for succinate dehydrogenase, 1.12 for cathepsin B, 0.607 for N-acetyl-β-D-glucosaminidase and 8.98 for gluta-thione-S-transferase. Adenylate cyclase response to parathormone (PTH), was similar in PTC and PT, but stimulated/basal ratios were higher in PT than in PTC. LLC-PK1 cells were stimulated by thyrocalcitonin(SCT),arginin-vasopressin (AVP) and PTH; stimulated/basal ratios ranked AVP>PTH>SCT. Differences between both types of cultures affect the choice of in vitro model for nephrotoxicity studies.