LONDON -- Standard&Poor's CreditWire 5/29/97 -- Standard&Poor's today has assigned its double-'Af' rating to Barclays Institutions, a short-term French mutual fund. The Barclays group (Barclays Bank PLC rated double-'A'/'A-1'-plus), is the promoter, investment adviser, and custodian for the fund. This rating reflects the credit quality of the portfolio's assets as well as the soundness of management, administration, and control procedures. The aim of Barclays Institutions is to offer a performance close to that available on the French money market, while maintaining the liquidity of the portfolio. The fund is aimed at investors who wish to invest large amounts for very short periods. The fund's assets consist mainly of commercial paper held directly or by way of