This proposal is in response to NSF Program Solicitation for Small Business Innovation Research (NSF 86-19) and is requesting support for Phase I research of this program. The overall goal of the proposal is to demonstrate the feasibility of measuring on-line the density of mammalian cells in bioreactor suspension cultures with the use of an optical sensor. The optical sensor is designed to be inserted into a reactor through a sampling port and will be connected to a Helium Neon laser source and a microprocessor. Research objectives for Phase I research are as follows: 1. Build a prototype optical sensor as described in the proposal. 2. Test the system using plastic microspheres as test materials. Different sizes of calibrating microspheres at various particle densities will be studied to optimize the technique for determination of particle concentration and mean particle size. 3. Obtain number density and size measurements from samples of mammalian cells and then optimize the instrument for the range of values which will be representative of bioreactor suspension cultures. If successful, this optical sensor would be a significant new tool in the analysis of mammalian cell cultures. The Panel reviewing this proposal rated it second of the 36 proposals reviewed and recommended it highly for funding. The investigators are considered to be very capable of carrying out the proposed research and I recommend funding of this proposal in accordance with the budget submitted with the proposal.