The initial reaction velocities (vi) of lactate dehydrogenase in hepatocytes, cardiac muscle fibres, skeletal (gastrocnemius) muscle fibres, gastric parietal cells, ductal epithelial and acinar cells of the parotid gland, and oocytes were determined, by computer-assisted image analysis, in unfixed sections of these tissues incubated at 37~ on substrate-containing agarose gel films. They were found to fit the equations vi=a~~ (equation 1) and v~-v =a2~ (equation 2) reported previously for mouse hepatocytes (Nakae & Stoward, 1993a, b), where v and ~ are, respectively, the gradients (or steady-state velocities) and the intercepts on the absorbance axis of the linear regression lines of the absorbance (A) of the final reaction product on incubation times between 1 and 3 rain, and a~ and a 2 are constants. Both equations I and 2 fitted the observed v i closely for mouse (a~ = 2.7, a 2 = 2.2) and human (a~ = 3.0, a 2 = 1.9) hepatocytes. However, equation 2 fitted the observed v i better than equation 1 for mouse cardiac muscle fibres (a2 = t.5), skeletal muscle fibres (a, = 1.2), gastric parietal cells (a2 = 1.7), acinar (a z = 1.4) and striated ductal (a 2--2.2) epithelial cells of the parotid gland, and oocytes (a2 = 1.6). The values of v~ calculated from the two equations agreed with the observed v~ to within about I1%. They ranged from 105 p, mole hydrogen equivalents/cm 3 cell/rain units in hepatocytes to 24 units in parotid acinar cells, but for other cell types they were between 46 and 6I units. These are all considerably higher than values reported previously.