A multicomponent assay several blood substrates is presented for human plasma using mid-infrared spectra recorded by the attenuated total reflection technique. Two different sample populations were analysed: a pool plasma spiked with different amounts of glucose and a hospital population of samples from 126 different patients. Partial least-squares was used for multivariate calibration based on spectral intervals in the fingerprint region selected for optimum prediction modeling. Different data, either plasma absorbance spectra with compensation of the water signal or single beam spectra, were considered. Results are reported for protein, glucose, cholesterol, triglycerides and urea.