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We investigate some properties of flat cosmological models with a ⌳ term that decreases with time as ⌳ ϰa Ϫm (a is the scale factor and m is a parameter 0рmϽ3). The models are equivalent to standard cosmology with matter and radiation plus an exotic fluid with the equation of state p x ϭ(m/3Ϫ1) x . We study the effect of the decaying ⌳ term on the cosmic microwave background anisotropy and by using a seminumeric method we compute the angular power spectrum ͑up to lϭ20) for different values of m and ⍀ m0 . We also investigate the constraints imposed on the models by the magnitude-redshift test in which high-redshift type Ia supernovae ͑SNe Ia͒ are used as standard candles. We obtain the 95.4%, 90%, and 68% confidence levels on the parameters m and ⍀ m0 and compare them with those arising from lensing statistics. Our analysis reveals that the SNe Ia constraints are stronger for low values of m and ⍀ m0 , while those from lensing statistics are more important for mտ1. Models with ⍀ m0 տ0.2 and mտ1.6 are in good agreement with the data. ͓S0556-2821͑97͒03620-5͔PACS number͑s͒: 98.80.Hw