The massive stellar content , the OB associations and the star clusters in an HST field in M101 = NGC 5457 are investigated .
A clustering algorithm yields 79 putative associations .
Their size distribution is similar to that found in the Magellanic Clouds , M31 and M33 , with an average size around 90 pc .
The V luminosity function for the stars contained within the associations has a slope d log N / d V = 0.60 \pm 0.05 , while an average reddening E ( B - V ) = 0.21 mag is measured .
The stellar content is further discussed by means of color-magnitude and color-color diagrams .
Ages are estimated using theoretical isochrones , and range between 3 and 14 Myr ( \pm 2 Myr ) .
We find a suggestion that the upper mass limit of the IMF for stars in OB associations in M101 may be quite high , contrary to some theoretical expectations that the mass limit should be lower in a high metallicity environment .
Forty-one star cluster candidates and two H II region core clusters are identified in the M101 field , and their integrated photometric properties are compared with the cluster system of the LMC and M33 .
Most of the M101 clusters probably belong to the class of young , populous star clusters such as are found in the LMC .
Red clusters are rare in this field .
In the Appendix the objective finding algorithm is applied to the brightest stars in the Large Magellanic Cloud .