Abstract: The invention contemplates instrumentation to enable performance of a plurality of automatic photometric analyses upon liquid samples, as in different analyses of such body fluids as blood, urine, etc. Prior to photometric evaluation, the liquid samples are automatically subjected to controlled doses of one or more reagents, and dwell times, as appropriate to the analysis to be performed at any given time. The particular automatic performance of a given analysis is supplied from an insertably replaceable holder which contains the reagents, dosage devices, and program-control coding unique to the particular analysis. Upon inserted replacement of the holder with another selected holder which is suitably encoded and equipped with reagents, the same instrument will serve for the handling of liquid samples unique to a different biological analysis.