A loading test on soft clay at Mastemyr, Norway, was carried out by the Norwegian Geotechnical Institute in 1968 to examine the build-up of displacements and pore pressures to failure. Although the test was heavily instrumented deductions from the measurements have not hitherto been published. The field observations of pore pressures and deformations are examined here for indications of the occurrence of yielding which is expected to be manifested in an increased rate of development of pore pressure and of strain with increase in loading. Information concerning the yielding of the clay was also obtained from the results of a programme of laboratory tests and this was used to produce a simple elastic-plastic model for the clay which could be used with various elastic analyses to study the manner in which yielding might be expected to develop in the clay under the loading. Factors which may influence the ability of simple models and the simple analyses to predict the development of yielding are discussed.